[CFA入门] 请教一个关于option的问题

各位兄弟姐妹，在option market and constract一章关于European option lower bound的推导，作者构建了一个如下portfolio( in which the option is combined with a long or short position in the stock and a pure discount bond)：

a long at the money european call option with exercise price X, expiring at time t =T.

a long discount bond priced to yield the risk free rate that pays X at option expiration.

a short position in one share of the underlying stock priced at S₀=X

the current value of this portfolio is c₀- S₀+X/(1+RFR)^T

^{我的问题是为什么是减掉那个S₀，能不能详细解释一下构建的这个portfolio，尤其是最后那个short  position，谢谢大家。}