单项选择题The cylindrical shape of a cactus reduces moisture loss.
单项选择题By 1900, many municipalities had begun to restrict the use of automobiles in order to en- sure pedestrian safety.
单项选择题Over thirty cities around the world boast more than five million residents.
单项选择题Proper exercise plays a significant role in the rehabilitation of patients with various back ailments.
单项选择题The polar regions are generally covered with ice and snow.