He tried his best to solve the problem, ______ difficult it was

下列各句中，加点的成语使用恰当的一项是（ ）

1Li Ping ______ a happyfamily.

(2013·哈尔滨四校高三统一检测)The old man often looked back on the past days ________ he worked hard to make a living.

The exam was very difficult but everyone ______ pass.