—When did you return to Nanjing last Friday?  

—It was not until midnight _______ it was snowing heavily.  [ ]

A. that

B. before

C. while

D. when

水泥厂对生料，石灰石等样品中二氧化硅的测定，分解试样的方法是（）

A、硫酸溶解

B、盐酸溶解

C、混合酸王水溶解

D、碳酸钠作熔剂；半熔融解；氢氧化钠熔融