式6ab与式-7ab的差为______．
根据题意列式得：6ab-（-7ab）=6ab+7ab=13ab．
________ a clerk work in a store? [ ]
A. Is
B. Are
C. does
D. Do
软膜胀形通常不选用（）为施力介质。
A.液体
B.钢材
C.气体
D.橡胶
