We must keep the noise under 50dB（分贝） here.The noise must __

We must keep the noise under 50dB（分贝） here.The noise must __ keep the noise We must keep our classroom clean The sentence stucture here The unpleasant noise must be immediately put an end to we must keep the ( )on the road we must keep the( ) on the road we must keep the balance of ( ).(自然界) we must keep( ) in the hospital We must ___(keep) the classroom clean. We must keep the window______(open). we must keep quiet at the library we must keep the reading room quiet