46. go to sea, by sea

go to sea 当海员，出航；
by sea 乘船，由海路；by the sea 在海边。

47. the doctor and teacher, the doctor and the teacher　

the doctor and teacher 指一个人，既是医生又是老师；
the doctor and the teacher 两个人，一个医生和一个老师。

48. in office, in the office　

in office 在职的，例：He is in office, not out of office.
in the office 在办公室里。

49. in bed, on the bed　

in bed 卧在床上，例：He is ill in bed.
on the bed 在床上，例：The book is on the bed.

50. in charge of, in the charge of　

in charge of 管理，负责照料，例：He is in charge of the matter.
in the charge of 由……照料，例：The matter is in the charge of her.