用括号中所给单词的适当形式填空。
（1） He is old enough. He can do it by ________ (he).
（2） It’s great ________(funny) to play with my pet.
（3） Please write a short passage about your dog at ________ (little) 100 words.
（4） Many ________ (visit) come to visit the Great Wall every year.
（5） Don’t climb over the wall. Look! It says, “It’s ________ (safe).”