棘轮效应 ratcheting
参考答案：
由于反应堆功率升降的反复，包壳的变形因燃料芯体反复膨胀而逐渐增大的现象。
设有关系R1和R2，经过关系运算得到结果S，则S是______。
A．一个关系
B．一个表单
C．一个数据库
D．一个数组
He is over fifty, but he looks as though he（）only in his thirties.
A.were
B.is
C.will be
D.has been
