题目

It’s very kind _________ you to help me carry the boxes since it is difficult _________ a person like you to carry so heavy things.  A. of, of                       B. of, for                   C. for, of                      D. for, for 答案：B    形成电流，电路中能产生持续电流的条件是：电路中要有     ，同时电路必须是     的．