题目

由NaHS、MgSO4、NaHSO3组成的混合物中，已知S元素的质量分数为a%，则O元素的质量分数为                                                     （    ） A、1.75a%         B、（100－1.75a）%      C、1.25a%       D、无法计算 答案：B—Lydia, what did our head-teacher say just now? —Every boy and every girl as well as the teachers who _____ to lead the group _____ asked to be at the school gate before 6:30 in the morning.A．is; isB．are; areC．are; isD．is; are