( ) it raining, we would have gone out.

If it had been…の倒置形であり、文脈上Hadは不成立なのでWereが妥当と見せかけて、ここは倒置構文のWere it not for等を想定しつつ、正解は仮定法過去完了の倒置でHad it not been…だが選択肢的に不完全。別の問題に変更。