I disagree with the answer choice A. One tree alone will not make it hard for other trees to grow...only when a stand has formed will all the sunlight be blocked....therefore, you could have one mature pine tree, then another one might have grown there 150 years later, and so on and so forth until you have your "stand.". nowhere does the passage allow you to conclude that all the trees grew together. How is that wrong?

Jorge, what you suggest may be true if someone intentionally plants a white pine individually. But that is knowledge that the passage does not provide and that we can't assume. The passage simply tells us that there is a white pine forest or collection of trees, and asks what the logical conclusion is give. The information that white pines don't grow in shades leads one to conclude logically that in a collection of white pine trees, they must have grown since around the same time.