60（）

单项选择题These are the greatest （） in human life.

A．situations
B．happenings
C．conditions
D．facts

单项选择题Aeroplanes usually do not need to climb quickly or fly high in the polar region because（）.

A．they do not land there
B．there are no clouds at all
C．they can cross the ice with special equipment
D．it is very cold