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) 正确答案为A。根据If you are silent for long periods of time, they will do their best to include you in their conversation.如果你沉默了很长一段时间，他们会尽他们所能让你加入到他们的谈话中。可知，如果你沉默了一段时间，美国人尽全力让你加入到谈话中，因此A项正确。
(2) 正确答案为B。根据However, if they disagree with what you are saying, they remain quiet. 如果他们不同意你的观点，他们会保持安静。可知，如果美国人不同意你的观点，他们不会与你争吵，而是会保持安静，因此B项正确。
(3) 正确答案为C。根据they may pat a child's head to show their affection for the child. 他们会拍小孩的头，以表达他们对孩子的喜爱。可知，美国人拍小孩子的头表达喜爱是正确的，因此C项正确。