请问这个b选项，为什么不能从买了长期债，r下降P上升，的角度来思考

如果用债券做carry trade，carry收益只包括息差收益，不含债券价格的涨跌收益。carry收益单指息差哈！

如果要考虑carry trade的total return的话，需要考虑：coupon收益与债券价差收益。

如下是Fabozzi教材的解释哈，CFA一级也有介绍：

For carry trades, the carry refers to the income generated from holding the bond, which is mainly the coupon income after accounting for the financing cost.

However, the overall profit or loss of a carry trade will include both the carry (coupon income) and the capital gains or losses from interest rate changes.