Official Explanation by mikemcgarry on his blog :
Split #1: SV Agreement. The pronoun “some” is the subject of the second half of the sentence. What is the implicit phrase: some of what? By “some”, this sentence means “some growth stocks”. Growth stocks are countable, and some of them would be plural, so we need the plural verb “are”. Choices (A) & (B) are incorrect.
Split #2: Mystery pronoun. A couple choices provided a subject to the verb “expected”: unfortunately, this is the pronoun “they” with no antecedent. The mysterious “they” who are expecting something are not identified. On the GMAT, this is unacceptable. Choices (A) & (D) make this mistake.
Split #3: speed. If P is moving faster than Q, how do we describe Q? It is unnecessarily wordy to say Q moves “with a speed less than” P, or, even worse, Q moves “with a speed that is less than” P. Those are the poor choices of (E) & (A) respectively. Choice (B) has “with a slower speed than” —- arguably, “slower speed” verges on the redundant. The most direct way to say this is: Q moves “more slowly than” P.
For all these reasons, (C) is perfectly correct and it is the only possible answer.