【推荐2】___________ (observe) the problem for many years, he announced that we are now facing the biggest environmental problem of our time.

【推荐3】When John Todd was a child, he loved to explore the woods around his house, ________ (observe) how nature solved problems. (所给词的适当形式填空)

【推荐1】用所给动词的正确形式填空
1. George returned after the war, only ________ (tell) that his wife had left him.
2. Heavy rain struck Guangzhou in May, ________ (cause) people traffic problems.
3. We mustn't pretend__________________(know)what we don't know.
4. She pretended__________________(read)a novel when I came into the room.
5. The boy pretended__________________(finish)his homework.

【推荐2】To achieve our goal, we need to think out of the box________________(explore) every possibility. (所给词的适当形式填空）

【推荐3】________(ensure) a correct understanding of the new regulations, we need to clarify some ambiguous points of view and put the system into practice. (所给词的适当形式填空)