[help] another old GMAT CR Q

A and D both involves in some new element, caregiver, who has never happeared in the argument.

C:connect the infants and smiling would be the answer to this question.

sorry, but I think I might have misunderstood the answer choice (C).
    

(C) The smiling response depends on an inborn trait determining a certain pattern of development.

does it mean the smiling response depends on an inborn trait which determines a certain pattern of development?

if it does, then i think it might weaken the hypotheses,

because normal children and children born deaf or deaf and blind begin social similing at almost the same time, smiling response happens no matter a child is normal or born deaf....