请根据括号中的中文提示、英文释义或句意，写出句中所缺单词，使句子通顺。
Do you have any difficulty            (解决) the problem?
Thank you for giving us some __________(建议) which are so valuable to us.
I doubt ____________(if) he will attend the meeting tomorrow.
Plenty of pop stars             (take part in) the presentation last week.
Mr. John received an expensive birthday gift from his friends, but he decides not to       it.