根据句意，用所给单词的适当形式填空。
【小题1】PM2.5 ________ (pollute) has become a serious problem which does harm to our health.
【小题2】When he woke up, he found himself tied to a tree, _________(able) to move.
【小题3】My dad has been to Chengdu on _______________ (busy) twice.
【小题4】She refused to join our ____________ (discuss) and finished her paper alone.