1.Circus tigers, although they have been tamed, can ___ attack their trainer.

A.unexpectedly

B.deliberately

C.reluctantly

D.subsequently

2.The tourist is prevented from entering a country if he does not have ___ passport.

A.an operative

B.a valid

C.an efficient

D.an effective

3.The plane is about to take off. Would you please ensure that your seat belt is securely ___?

A.locked

B.bound

C.tied

D.fastened

4.Though working for the old, he paid ___ visits to his own parents.

A.scare

B.rare

C.insufficient

D.inadequate

5.Stressful environments lead to unhealthy behaviors such as poor eating habits, which ___ increase the risk of heart disease.

A.in turn

B.in return

C.by chance

D.by turns

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