The issue with (C) is the modifier at the beginning, "on Earth." It's not clear what it refers to--is it referring to the rate at which trees grow on Earth (as opposed to how well they grow on venus)? The belief about sunspots on Earth (as opposed to what Martians believe)? Or the indicators of sunspot cycles on Earth (as opposed to visiting to travel the sun)? The third is clearly the only logical meaning, but grammatically, choice C could be interpreted as any of the three. By moving the modifier to be near the word "indications," choice (D) and (E) avoid that ambiguity.