但你只能做多中国的股票)”
Johnny继续绷着脸问问题：“It is good that you realize that before you start working. What are the other assumptions?（在开始工作之前就意识到这一点很好。其他假设是什么）”
晓兮答道：“There are five other assumptions. First， the risk free rate is constant. Second， the stock price follows geometric Brownian motions with constant drift and volatility.（还有其他五个假设。首先，无风险利率是恒定的。其次，股票价格遵循具有恒定漂移和波动性的几何布朗运动）”
还没等晓兮说完，Johnny就打断了她：“All right. You don’t need to say the rest. I understand you know them all. Now let us assume that it is okay to use the Black-Scholes formula. Can you calculate the option price now?（好的。你不需要说剩下的。我知道你知道所有的假设。现在让我们假设可以使用 Black-Scholes公式。你能计算出期权价格吗）”
晓兮马上开始在白纸上写，她开始计算d1，各个部分都很简单，除了那个自然对数，等于1这大家都知道，但是并不是常用的，晓兮一下子并不知道是多少。这时晓兮抬头看了Johnny一眼，发现Johnny也一直都在看着他。
Johnny面无表情，但是语气严厉：“You must have taken your calculus class. Don’t tell me you don’t know how to calculate that.（你一定上过微积分课。别告诉我你不知道怎么计算）”
晓兮脑海中出现的方法只有泰勒级数，于是她就开始套公式开始算。当她把d1算出来时，Johnny突然说道：“You can stop here. I have got an idea of you math skill. Let us move on to the next question. Can you explain to me what is delta?（你可以在这里停下来。我对你的数学能力已经了解啦。让我们继续下一个问题。你能给我解释一下什么是delta吗）”
这个晓兮当然知道：“Delta measures the sensitivity of the option price to the changes in the underlying stock prices. For example， if a call option’s delta is 0.4， it means if the underlying stock price increases by 1， the options prices will increase by 0.4.（Delta衡量期权价格对标的股票价格变化的敏感度。例如，如果看涨期权的 delta为 0.4，则意味着如果标的股票价格上涨 1元，则期权价格将上涨 0.4元。）”
Johnny还是一贯的紧追不舍：“why do I bother with delta? Why do I need it?（为什么我需要delta）”
晓兮答道：“If you have an option and want to hedge it， delta tells you how many shares of stock you need.（如果您有期权并想对其进行对冲，delta会告诉您需要多少股股票）”
Johnny继续问道：“Can you estimate the delta of the index option I specified just now?（你能估算一下我刚才说的指数期权的delta吗）”
晓兮答道：“Delta is N(d1). N(d1) is N(-0.933). We know that 1 standard deviation covers 68%. So N(-1) is about 1 minus 0.68 divided by 2. It is 0