Some experts advise that steps ________ to protect the river at once, otherwise it's dangerous for people to drink the water.
A.being takenB.be takenC.were takenD.had been taken在线课程B
解析：
advise表"建议，劝告"，后接宾语从句时从句谓语动词应用"should+动词原形"形式，表示的是虚拟语气，其中should还可省略。steps和take之间是被动关系，因此正确答案为B。