【英语单选】高手进!1with nobody living with him he began to regret_of

【英语单选】高手进!
1with nobody living with him he began to regret_of such a large house
A purchasing B the purchase
为什么不选A regret doing 后悔做某事
2 Do not answer me with a simple "yes" or "no"
Explain ________ possible．
A. whoever
B. whenever
C. however
D. whatever
选项是怎么逐一排除的 求赐教!