1. It was his own fault, but I couldn't help......sorry for him  a. Feeling  b. To feel  c. Having felt  d. To have felt                                                                             2. The medical authorities watned everyone not to drink the water without first......it.  a. Being boiled  b. Boiling  c. Having boiled   d. Having to boil                3. Most people talk too much and don't realize how important ......is  A. To be listening  B. To be listened   c. Have listened   d. Listening                                             4. If you've finished....the dictionary, I'd like to borrow it.  A. To have used   b. To use       c. Using         d. Having used   Giải thích giúp mình luôn nha