磁电式仪表的线圈常常用铝框做骨架，把线圈绕在铝框上，而不用塑料做骨架是因为[ ]

A．塑料材料的坚硬程度达不到要求  

B．塑料是绝缘的，对线圈的绝缘产生不良影响

C．铝框是导体，在铝框和指针一块摆动过程中会产生涡流，使指针很快停止摆动

D．铝框的质量比塑料框的质量大

The workers are repairing the windows  _______  glasses were broken by strong wind last night. [ ]

A. their

B. where

C. whose

D. that's