根据句意，用括号内所给单词的适当形式填空。
The problem is so easy that I have no difficulty        (explain) it to my classmates.
If you tell him the            (true)，he will be angry.
You may ask your teacher for help. He will give you a few        (value) suggestions.
People in the past suffered from hunger and         (ill).
I invited Sally to my party, but she refused         (accept) my invitation.