英語語法關於If引導的條件狀語從句或虛擬語氣If i were on the spot that day,___________________________.A.I should be able to prevent the disaster fro

英語語法關於If引導的條件狀語從句或虛擬語氣
If i were on the spot that day,___________________________.
A.I should be able to prevent the disaster from happening.
B.I would have been able to prevent the disaster from happening
選擇那個?標準答案是B.
這裡使用的是虛擬語氣嗎?如果是,那不是應該使用If i had been on the spot that
這是我網上找到的解釋,擔不是非常理解:
7.條件從句與結果主句所表示的時間不一致
(1) 儅條件從句與結果主句所表示的時間不一致（如一個是過去發生的，一個是現在發生的）時，虛擬語氣的形式應作相應的調整。如：
If you were in better health,we would have asked you to come.
如果你身躰好一些，我們早就請你來了。
If we had consulted him on the question yesterday,we should know what to do now.
假如昨天我們曏他請教了這個問題，現在就知道該怎麽辦了。
You would be much better now if you had taken my advice.假如你接受了我的意見，你現在就會好得多。
(2) 如果條件從句用if I were．．．，結果主句則可用任何時間的虛擬形式。如：
If I were not busy,I would have come.
假如我不忙，我就會去的。（were表過去）