One Reason an English-Speaking Adult May Have Difficulty Pronouncing Russian

One reason an English-speaking adult may have difficulty pronouncing Russian words is that

A) the vocal tracts of English- and Russian-speaking people develop differently in response to the demands of the two languages.
B) although English and Russian have very similar morphemes, their phonemic inventories are very different.
C) although English and Russian have very similar phonemes, their morphemic inventories are very different.
D) after the babbling stage, a child who hears only English stops uttering other phonemes.