What is the average salinity of ocean water and which salt contributes the largest share of it?

The correct answer is D, only II. Statement I is wrong because the equatorial belt receives very heavy rainfall and has high cloud cover, so the surface water there is diluted; the highest salinity in the open ocean is recorded in the subtropical high pressure belts, roughly between twenty and thirty degrees of latitude, where skies are clear, evaporation is strong and rainfall is slight. Statement II is correct, because the northern hemisphere, with its enclosed and partly landlocked seas and its narrower expanse of cold southern water, has a higher average surface salinity than the southern hemisphere, where the vast Southern Ocean is fed by melting ice. Option A is wrong because statement II is true. Option B is wrong because statement I is false. Option C is wrong for the same reason, since it accepts the equatorial claim.

The correct answer is B, the Mariana Trench, in the western Pacific Ocean, whose Challenger Deep reaches a depth of about eleven kilometres and is the deepest known point on the earth's surface. Option A, the Java or Sunda Trench, is wrong because it is the deepest trench of the Indian Ocean, not of the world. Option C, the Puerto Rico Trench, is wrong because it is the deepest part of the Atlantic Ocean. Option D, the Tonga Trench, is wrong although it is indeed one of the deepest trenches of the Pacific, second only to the Mariana. Trenches are the narrow, steep-sided deeps that lie beyond 6,000 metres, they form where one plate descends beneath another, and most of the world's trenches are found around the rim of the Pacific.

The correct answer is A, the continental shelf. It is the gently sloping, shallow margin of the continent, extending to a depth of about 200 metres with a gradient of one degree or less and an average width of about seventy to eighty kilometres, and it makes up roughly 7.5 per cent of the ocean area. Sunlight reaches its floor, so plankton grow in abundance and the great fisheries of the world lie over shelves; most offshore oil and gas also come from them. Option B, the continental slope, is wrong because it is steep and deep and marks the true edge of the continent. Option C, the abyssal plain, is wrong because it lies between 3,000 and 6,000 metres and yields only manganese nodules and fine oozes. Option D, the trench, is wrong because it is the deepest and least exploited part of the sea.

The correct answer is B, new moon and full moon. On these days the sun, the moon and the earth lie in a straight line, a position called syzygy, so the pulls of the sun and the moon act together and the tidal range is at its greatest; spring tides therefore occur twice in a lunar month and have nothing to do with the spring season. Option A is wrong because the first and third quarters give neap tides, when the sun and the moon pull at right angles to each other and partly cancel out, producing the smallest range of the month. Options C and D are wrong because they take only half of the correct answer; the alignment is equally effective whether the moon is between the earth and the sun or on the far side of the earth.

The correct answer is B, the Bay of Fundy, between Nova Scotia and New Brunswick in Canada, where the funnel shape of the bay and the length of the inlet make the incoming tide pile up to a range of the order of fifteen to sixteen metres. Option A, the Gulf of Kutch, is wrong as a world answer, although it does have one of the highest ranges in India along with the Gulf of Khambhat, which is why tidal power has long been proposed there. Option C, the Strait of Gibraltar, is wrong because the Mediterranean beyond it is almost tideless, its narrow entrance letting in very little tidal water. Option D, the Gulf of Mexico, is wrong because it has an unusually small range and chiefly diurnal tides.