When ________ what his problem was, the boy lowered his head and kept silent.[ ]

A. asking  

B. asked

C. having asked  

D. having been asked

设NA为阿伏加德罗常数的值。下列叙述正确的是

A．5．6g铁和6．4g铜分别与0．1mol氯气完全反应，转移的电子数相等

B．0．5molO₃与11．2LO₂所含的分子数一定相等

C．等质量的 ¹⁴NO和 ¹³CO气体中含有的中子数相等

D．常温下，5．6g铁钉与足量浓硝酸反应，转移的电子数为0．3NA