Harrison Ford is thought to be one of the few movie stars ____ as a carpenter before.

A．to work

B．to be working

C．to have worked

D．to have been working

关于磁感应强度，下列说法正确的是（　　）

A．磁感应强度只能描述磁场的强弱

B．通电导线所受磁场力为零，该处磁感应强度一定为零

C．通电导线所受磁场力为零，该处磁感应强度不一定为零

D．放置在磁场中lm的导线，通过lA的电流，受到的力为1N时，该处磁感应强度就是1T