Definition: In this task, You are given an open-domain question that can be answered based on factual information. Your task is to provide \*short\* answer (in a few words only) for the given question. The short answer can be one or more entities or it can also be boolean \*yes\* or \*no\*.

Positive Example 1 -
Input: when are hops added to the brewing process?
Output: The boiling process

Positive Example 2 -
Input: who played will on as the world turns?
Output: Jesse Soffer

Negative Example 1 -
Input: where do dust storms occur in the US?
Output: 

Negative Example 2 -
Input: when did the watts riot start and end?
Output: watts riot

Now complete the following example -
Input: the number of membranes surrounding a mitochondrion is

Output: two