Which of the following statements about meridians of longitude is correct?

The correct answer is B, 82 degrees 30 minutes east. This meridian passes near Mirzapur in Uttar Pradesh and runs roughly through the middle of the country, so it was adopted as the standard meridian of India. Its local time, which is five hours and thirty minutes ahead of Greenwich Mean Time, is kept as Indian Standard Time everywhere from Gujarat to Arunachal Pradesh.

Option A, 75 degrees east, passes through the western part of the country and is often offered because it sounds like a round figure. Option C, 88 degrees 30 minutes east, lies close to Kolkata, which once kept its own local time before standardisation. Option D, 90 degrees east, is the standard meridian used by Bangladesh, six hours ahead of Greenwich. The half hour in the Indian offset comes directly from the thirty minutes in the chosen meridian, since half a degree equals two minutes of time.

The correct answer is B, four minutes. The earth completes one rotation of 360 degrees in 24 hours, that is 1,440 minutes, and dividing 1,440 by 360 gives four minutes for every degree of longitude.

Option C, fifteen minutes, reverses the relationship that candidates most often remember, which is that the earth turns fifteen degrees in one hour; fifteen is the number of degrees, not of minutes. Option D, one hour, is the time for fifteen degrees, not for one. Option A, one minute, would mean a full rotation in six hours, which is plainly wrong. Every numerical question in this chapter rests on this single conversion, so keep both forms ready: one degree equals four minutes, and fifteen degrees equal one hour. The half degree that appears in Indian Standard Time therefore accounts for exactly two minutes.

The correct answer is C, Odisha. The Tropic of Cancer passes through eight states, namely Gujarat, Rajasthan, Madhya Pradesh, Chhattisgarh, Jharkhand, West Bengal, Tripura and Mizoram. Odisha lies to the south of that line and is therefore wholly within the tropical belt.

Option A, Jharkhand, and option B, Chhattisgarh, both lie on the line in central and eastern India, and option D, Tripura, is one of the two north-eastern states it crosses, the other being Mizoram. Examiners set this question in both directions, sometimes asking which state the line passes through and sometimes which it does not, and the usual wrong options are Odisha and Bihar, both of which lie just outside it. A simple way to hold the list is to run from west to east: Gujarat, Rajasthan, Madhya Pradesh, Chhattisgarh, Jharkhand, West Bengal, Tripura, Mizoram.

The correct answer is A, the equator. A great circle is one whose plane passes through the centre of the sphere and which therefore divides it into two equal halves. Among the parallels only the equator satisfies this, which is why it separates the northern and southern hemispheres and is the longest parallel of all.

Option B, the Tropic of Cancer, and option D, the Tropic of Capricorn, lie 23.5 degrees away from the equator, so their planes are offset from the centre and the circles they form are smaller. Option C, the Arctic Circle, at 66.5 degrees north, is smaller still, because parallels shrink steadily towards the poles. Meridians behave differently: every meridian and its opposite meridian together make a full great circle, which is why the shortest air route between two distant cities follows a great circle and often looks curved on a flat map.

The correct answer is B, Greenwich, the site of the old Royal Observatory near London. The meridian running through it was accepted as the zero meridian, from which all longitudes are counted east and west, and its local time is Greenwich Mean Time, the reference for the world's standard times.

Option A, Paris, is a strong distractor because France long used its own prime meridian through the Paris Observatory before the international agreement. Option C, Rome, and option D, Washington, were never adopted as the zero meridian, although several countries did use national meridians of their own in the nineteenth century. Remember the pair of extremes for this chapter: the equator is the reference for latitude and the Greenwich meridian is the reference for longitude, and the meridian directly opposite Greenwich, at 180 degrees, is the one the International Date Line broadly follows.