如图所示的滑动变阻器的四种接法中，当滑片P 向右移动时使电路的电阻变小的接法是[ ]
A．
B．
C．
D．
AD
If the weather had been better, we could have had a picnic. But it _____ all day. [ ]
A. rained
B. rains
C. has rained
D. is raining
流感病毒最易变异的结构是（）
A.甲型流感病毒的HA
B.乙型流感病毒的HA
C.核蛋白
D.M蛋白
E.RNA多聚酶
