He became a teacher when he was __.A on his twenties B in his twentiesC in his twenty D at his twenties选哪个,

He became a teacher when he was __.A on his twenties B in his twentiesC in his twenty D at his twenties选哪个,
He became a teacher when he was __.
A on his twenties B in his twenties
C in his twenty D at his twenties
选哪个,

He became a teacher when he was __.A on his twenties B in his twentiesC in his twenty D at his twenties选哪个,
B 在他20来岁的时候 要用in 多少来岁要复数