27. Great changes _______in our city in the past twenty years.

A．have been taken place

B．have taken place

C．took place

D．were taken place

永久气体气瓶充装系统用的压力表，精度应不低于( )级，表盘直径应不小于( )mm。

A．1.5；150

B．1.0；100

C．1.0；200

D．1.5：100