Explanation

@virgiebeatriz, Happy to help. Let's first take a look at the stimulus before looking at (B) and (D). The stimulus tells us that both babies who can hear and babies who are deaf exhibit signs of babbling. For hearing babies, they babble with their mouths/voices, and for deaf babies, they babble with their hands. In both cases, babbling functions as a precursor to either speaking or using sign language. The question says, "The information above, if accurate, can best be used as evidence against which one of the following hypotheses?" We want to choose the answer choice that states something that goes against the phenomenon of babbling and its relationship to language. (D) says, "In babbling, babies are unaware that the sound or gesture combinations they use can be employed in a purposive way." The problem with (D) is that we don't have any information from the stimulus that discusses what babies are aware or unaware of in terms of language. All we know is how they physically behave, but we have no idea if they're aware that the sound or gesture combinations they're using are for a purpose. The evidence in the stimulus and (D) aren't necessarily at odds with one another. As a result, we can't say that the evidence in the stimulus can be used as evidence against (D)'s claim, so (D) isn't the correct choice. (B) says, "The development of language competency in babies depends primarily on the physical maturation of the vocal tract, a process that requires speech oriented vocal activity." We're told in the stimulus that deaf babies begin to babble in signs using their hands. This implies that babbling is a precursor to communicating through sign language, just as a baby babbling with his or her voice is a precursor to speaking. What this means is that speaking is not required for developing language competency. (B) says that speech oriented vocal activity is required for developing language competency, and the stimulus provides us with evidence that goes directly against this statement, so (B) is the correct answer choice. Does this make sense? Do you see how we can categorically eliminate (D) from contention? I would argue that in this question, there are not two decent answers—there's only one good answer choice, and all of the other choices have flaws that make them incorrect. Let us know if you have any more questions!

Okay, I see your point. I think I was reading a little more than what the passage was saying. Just to make sure, you're saying that had the passage explicitly stated that babies knew what they were doing was for a purpose, answer choice (D) would also have been an acceptable answer, correct?

@virgiebeatriz, That's correct. However, since the passage does not provide us with such information, we cannot say for sure that the stimulus provides us with information that goes against (D). Hope this helps. Let us know if you have any other questions!