We haven’t seen each other for 20 years, but I recognized him _____ I saw him．Which of the following is wrong ?

We must do something to _____ the factories sending outpoisonous gases so as to keep the city_____.[ ]

“What a pity! Two thirds of the apples I’ve just bought are ___.” He said.

Happiness and success often come to those who are good at recognizing their own .

The actor isn’t as young as he appears_____, he is already in his fifties