A、The right hand. B、The left hand. C、Both hands. D、It depends. B原文提到，婴儿16周大时，主要使用左手进行触摸，因此B正确。

Approximately 90% of people in the world are naturally right-handed. Why is this so, and are we born one way or the other? The answer to this question is rather complicated.
    In babies and young children, no single side becomes dominant until around the age of eight. At 12 weeks, babies usually use both hands equally, but by 16 weeks, they mostly use the left hand for touching. By 24 weeks, they have changed again and start using both hands. Then at 28 weeks, they become one-handed again, although this time it is the right hand that is used more. At 32 weeks, they start using both hands again. When they reach the age of 36 weeks, there is another change, with most babies now preferring to use the left hand. Between 40 and 44 weeks, the right hand is once again more used. At 48 weeks, babies switch to using their left hands again, and then between 52 and 56 weeks, the right hand takes over.
    There are further changes still. At 80 weeks, the right hand loses control, and both are used again equally. When the young child reaches the age of two, the right hand takes over again, but between two and a half and three years, both hands are used equally. Things finally become stable at around four years and stay the same until, by the age of eight, one hand is strongly dominant over the other.
19. Which hand do babies use most for touching when they are at 16 weeks?
20. Until how old will a child’s one hand be strongly dominant over the other?
21. What do we learn about babies’ using their hands?