Received a PM asking me to respond. I'm not really happy with this question. I'm curious where it's from?

The best explanation that I can give you is that, in this instance, we are allowed to assume that, because we are making a comparison and saying these two comets are doing / will do the same thing, we're allowed to assume that the second instance of "will" already implies "will do the same thing as described for the first comet." The addition of "do" in E, therefore, is unnecessary / not concise.

But I can't recall any official question that has something that... lame, frankly, as the only problem with a wrong answer. It's the kind of thing that a native speaker will say "You just wouldn't say it that way" and the test tries to avoid that nowadays - doesn't want to introduce too much of an artificial bias against non-native speakers. The rest of it looks pretty well-written though. Maybe this is an old paper question from a loooong time ago, when the test wasn't as well-written as it is now?