Storage of biological tissues such as blood, organs, semen requires low temperature which is achieved upon the usage of

The correct answer is A, Liquid nitrogen. Liquid nitrogen boils at about −196 °C, cold enough to stop almost all biological activity, and it is cheap, inert and safe to handle. Keeping living cells at such low temperatures is called cryopreservation. Sperm, eggs, embryos, stem cells, cord blood and some tissues are stored in tanks of liquid nitrogen for years and can be revived later; the same method helps animal breeding programmes store bull semen. Because nitrogen is unreactive, it does not damage the samples. Option B is wrong because solid carbon dioxide, or dry ice, sublimes at only about −78 °C, which is used for shipping frozen goods but is not cold enough for long-term storage. Option C is wrong because liquid air contains liquid oxygen, which makes it a fire hazard and unsuitable for routine storage. Option D is wrong because liquid helium, though colder, is very costly and is used mainly to cool superconducting magnets such as those in MRI machines. Exam tip: liquid nitrogen −196 °C = cryopreservation; dry ice −78 °C; liquid helium = MRI magnets.