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