冬天北方农村的菜窖里放几桶水，菜就不易冻坏，这是因为水 时会 热，因此窖内温度不致太 。
凝固，放，低
It is believed that a new kind of drug ______ by the scientists and they are hopeful that they
will succeed in a couple of months. [ ]
A. has been developed
B. is being developed
C. is developing
D. has developed
心房颤动的心电图中，f波频率在
A．100～200次/分
B．150～250次/分
C．200～250次/分
D．250～300次/分
E．350～600次/分
