The president expressed his satisfaction with the co-operation,       that he hoped to have another one.

A．adding

B．added

C．to add

D．having added

下列关于电流形成磁场的叙述，说法正确的是（）

A．电流形成的磁场与电流方向平行；

B．电流从一根导体上通过时，用右手定则确定磁场方向 

C．通电导体周围的磁场强度与电流大小无关；

D．通电导体周围的磁场强度与电流大小无关