A与B是相关联的量，3：A=B：7，则A与B成______比例．
因为3：A=B：7，
则：AB=21（一定）；
所以A与B成反比例．
故答案为：反．
-He seems to be doing nothing, but he is just waiting for his chance.
A．actually
B．gradually
C．frequently
D．certainly
按体积分数计算，氧气在空气中约占（ ）
A．78%
B．21%
C．0.94%
D．0.03%
