Of the two brothers, Bill is ___ younger one, and he is ____ quiet boy, one that most adults like very much.

A．the; a

B．a ; the

C．a; one

D．the ; the

汽轮机及发电机轴系的临界转速与额定转速应有（）、以防止机组启动升速、危急保安器升速试验时出现有害影响。