Whenmakingmoderncameras,peoplebeganto_______plasticsformetal.
Afterworkingfortwentyhourswithoutanyrest,thedoctorswere_______.
ProfessorSmithisalsothe_______oftheinternationalprogramoffice.Ifyouhaveanyproblemwhenyoustudyhere,youmaygotohimforhelp.
Asateacher,youshouldnot_______thestudentsfromaskingquestionsinclass.