英语阅读理解：请问这题目中B为什么不能选？答案是D

英语阅读理解：请问这题目中B为什么不能选？答案是D
原文：What is happening when we give gifts? More importantly, we are exchanging gifts. If someone gives me a gift for my birthday, I know that I am usually expected to give one on his or her next birthday. A gift builds up or confirms a social obligation.
From thesecond paragraph we can infer that ________.
A. gift giving is proven to be valuable B. it is our social duty to givegifts
C. gift giving is a wasteful practice D.gift giving is a two-way social activity