题目

—Why is Maria so nervous?  Because she doesn't know ________at the big party．         A．to wear what    B．to wear how   C．what to wear D．how to wear     答案：C一瓶氧气的密度为5kg/m3，对病人供氧时用去了一半，则瓶内剩余氧气的密度为    kg/m3．