Which Response Is Not a Challenge Engineers Faced in Designing

Which response is not a challenge engineers faced in designing automated speech recognition software?

A) The stream of speech sounds proceeds without many silences, making it hard to decide when the signal has ended one word and begun the next word.
B) Individual speakers vary widely in how they pronounce phonemes.
C) Even a voice-activated toy dog must understand the meaning of individual words if it is to respond differently to each of them.
D) Background noise overlaps with the kinds of sounds present in the speech signal, making it unclear which source produced which component of the sound stream.