题目

She doesn't know _________________ made her daughter so sad this morning. A. what was it    B. why it was that C. how that was   D. what it was that 答案：D. 解析: 强调句的特殊疑问句用作宾语从句时用陈述语序宾语从句缺主语故用what 引导如图，⊙O1、⊙O2的直径分别为2cm和4cm，现将⊙O1向⊙O2平移，当O1O2=________cm时，⊙O1与⊙O2相切．