Why would you pay--if the share is selling for $25,

假定股价在25美元
why would I pay $30 for the right to buy at that $20?

我为何要花30美元买以20美元购入股票的权利
Obviously it's ridiculous;

显然这很荒谬
the stock itself is an option to buy the share at a $0 exercise price,

股票本身便是一种执行价格为0元的期权
so it has to be worse to have a positive exercise price.

因此执行价格为正会比较糟糕
Now, I wanted to stress the put-call parity relation

现在我想重申一下看跌-看涨期权平价关系
and this is another arbitrage thing.

这是另一个套利问题
The put option price--

看跌期权的价格
arbitrage--the absence of arbitrage opportunities implies

套利套利机会的存在暗示着
that the put option price minus the call option price

看跌期权价格减看涨期权价格
equals the present value of the strike price--

等于执行价格的现值
that's discounting it from this exercise date to the present--

该值是由到期日折现而来