在生态系统中各种生物的_______和所占的_______总是维持在相对稳定的状态，这种现象叫_________。
数量，比例，生态平衡
Tom did all _______ he could to get on well _______ his classmates. [ ]
A. which; with
B. that; /
C. that; with
D. /; /
闸阀与截止阀相比，其（）。
A．水流阻力较小
B．严密性好
C．阀体比较短
D．在开启和关闭时较省力
