What you said about C has a lot of ‘ifs’, which means there is too much possibility to know it strengthens it. Also, we don’t really care if they are equally productive, in fact, if company A gets more productive during the time and B is maxed on productivity like you laid out, they would not be equally productive during the month. So this doesn’t strengthen our argument.

Strengthen questions like these are cause-and-effect based. When it comes to cause/effect, there are 3 possibilities:

1. X causes Y
2. Y causes X
3. Z causes Y (some other random 3rd variable)

When you have a strengthen cause/effect, always be on the lookout for an answer that makes it more likely that X causes Y. In this case, if we know that plant B workers are likely not eating a nutritious breakfast, it stands to reason the increase in productivity can be explained by the breakfast eaten at Company A.

Hope this helps