Mr. Li has ordered ___ for the school lab.

As the quality of the city’s air continues to give rise to ______, the residents are encouraged to set off fewer fireworks.

As a doctor , it’s against my ______ to accept gifts from patients. 

As more and more cars are produced and used, so the _____ from their exhaust-pipes contains an even larger volume of poisonous gas.

He was faced with the    of whether or not to return to his country after he got the doctor's degree in America．