She’ll spend as much time as she can      to improve her spoken English.

A．speak English

B．to speak English

C．speaking English

D．speaks English

下列选项种，不会造成银镀层发暗、有斑点的是（）。

A.银含量低

B.游离氰化物过少

C.有机杂质污染

D.银阳极不纯