题目

She’s already two hours late. What ______ to her? A. can have happened              B. may have happened  C. should have happened           D. must happen 答案：A 解析: can表推测，只用于否定句和疑问句中。某元素的一种同位素X的原子质量数为A，含N个中子，它与1  H原子组成HmX分子，在agHmX中所含电子的物质的量是 A．(A－N＋m) mol                     B．(A－N) mol C．(A－N) mol                D．(A－N＋m) mol