问题如下：
What is (a) a position limit and (b) an exercise limit in the options market? Why are they used?
选项：
解释：
The position limit is a limit on the size of the position that an investor (or group of investors acting together) can have in option contracts. For this purpose, the positions in long calls and short puts are summed and the positions in long puts and short calls are summed. An exercise limit is a limit on the number of options that can be exercised within a five-day period. The purpose of the limits is to prevent an investor (or group of investors) from unduly influencing the market.
For this purpose, the positions in long calls and short puts are summed and the positions in long puts and short calls are summed. 如果按照单边计算，为什么不是long call 和 short call呢？
