To improne the pmlity of our products we uaked for nuggratior a                 had used the producu.

A．whacver

B．who

C．whichever

D．which

从字符串S("abcdefg")中返回子串B("cd")的正确表达式是______。

A．Mid(S,3,2)

B．Right(Left(S,4),2)

C．Left(Right(S,5),2)

D．以上都可以