They don't feel comfortable when (20) still keep treating them like little

Mark the letter A, B, C, or D to indicate the correct option that best fits each of the numbered blanks from 18 to 23.

Throughout history, there have always been conflicts between parents and their teenage children. Here are some of the main reasons and explanations.
No matter how old their teenage children are, most parents still treat them like small kids. As they try to help their children to discover the surrounding world, parents strongly believe they know (18) ______. However, (19) _______, they want to be more independent, create their own opinions, and make their own decisions. They don't feel comfortable when (20) _______ still keep treating them like little kids.
One common area of conflict is the clothes children want to wear. Parents may think that these clothes break rules and norms of society, or (21) _______. Another source of conflict is the way children spend their free time. Parents may think that their children should (22) ______ rather than playing computer games or chatting online. But children do not always see things the way their parents do.
Conflicts also arise from different interests between parents and their children. Some parents may try to impose their choices of university or career (23) _______. Open communication can really help to create mutual trust and understanding between parents and their teenage children.