After six hours the plants were frozen in liquid nitrogen, before being put onto photographic film to make the autoradiograph. Suggest why the plant must be frozen quickly

Explanation: I suggest:  A freeze occurs when the temperature drops below 32 degrees, and frost can happen when the air temperature (which is measured about six feet above the ground) is up to about 36 degrees. If part of your yard is lower than the rest, it’ll probably be coldest there since cold air is heavier and sinks down.

not too sure but maybe in the hot springs.

yes, because even though a theory has a  lot of evidence to support it, it is not always true