015.
The distance at which two people stand when having a conversation varies from culture to culture. Studies have shown that most Americans feel uncomfortable if they stand too close to others when talking. They feel most comfortable at a distance of about 50 centimeters.
Another social custom is that Americans, like many other people, often express themselves with their hands when talking. They may show a conversation or they may pat a child's head to show their affection for the child.
Many Americans find silence uncomfortable. They will talk continually to fill any pause in a conversation even if it lasts only for a moment. If you are silent for long periods of time, they will do their best to include you in their conversation. However, if they disagree with what you are saying, they remain quiet. It only means that they consider it impolite to argue further.

答案解析：
(1) 正確答案為A。根據If you are silent for long periods of time, they will do their best to include you in their conversation.如果你沉默了很長一段時間，他們會盡他們所能讓你加入到他們的談話中。可知，如果你沉默了一段時間，美國人盡全力讓你加入到談話中，因此A項正確。
(2) 正確答案為B。根據However, if they disagree with what you are saying, they remain quiet. 如果他們不同意你的觀點，他們會保持安靜。可知，如果美國人不同意你的觀點，他們不會與你爭吵，而是會保持安靜，因此B項正確。
(3) 正確答案為C。根據they may pat a child's head to show their affection for the child. 他們會拍小孩的頭，以表達他們對孩子的喜愛。可知，美國人拍小孩子的頭表達喜愛是正確的，因此C項正確。