It is fairly common in Africa for     a group of musicians surrounded by others who also join in the performances.

A．there are

B．there being

C．there to be

D．there be

长方形的一边等于2a+3b,另一边比它小a-b,则长方形的周长为             （   ）

A．3a+2b

B．a+4b

C．6a+14b

D．10a+10b