问题如下：
A trader feels that there will be a big jump in a stock price but is uncertain of the direction. Identify three trading strategies that the investor can implement to reflect his or her views.
选项：
解释：
The trader can try a short butterfly spread, a short calendar spread, a strangle, or a straddle.
现在讲义里好像没有short calendar spread？只看到有calendar spread 和 reverse calendar spread。
calendar spread是 ：short call (short-maturity)+ long call (long-maturity)
reverse calendar spread是：short put(short-maturity)+ long put(long-maturity)
那么short calendar spread是什么？
