At a supermarket in County Durham, England, the workers are getting ready for some lessons in sign language . They are excited to meet their teacher -- two-year-old Diana Graham. The little girl will teach them how to communicate with deaf customers . Diana is a wonder child. She began to use sign language to "speak" to her deaf mother before she said her first word. So whenever the phone rings or there's a knock at the door, her mother can be quickly told by her. Diana understands the sign language well. Her mother even reads story books to her in sign language. Her grandmother said, "Diana started signing at about five months. She is so clever for her age."The supermarket officials couldn't think of a better sign language teacher for their workers. So they asked Diana to give some lessons to the workers. They hoped that her lessons would help the workers communicate better with deaf customers during their shopping. Diana understands the sign language well because _ . A) she is old enough B) she is clever C) her grandmother teaches her D) her mother communicates with her in it
A:
D