题目

锂电池可用作心脏起搏器的电源。已知一种锂原子核内含有3个质子和4个中子， 则该锂原子的核外电子数为（ ） A . 1 B . 4 C . 3 D . 7 答案：C________, he kept on ________ the box. An hour later he succeeded. A. No matter how hard it was; trying opening B. However it was hard; trying to open C. No matter how it was hard; to try to open D. However hard it was; trying to open