21. A. Because birth-control was developed, women could delay having children.
=> Because of + noun/ V-ing = Because + clause: bởi vì
22. D. A & B are correct.
=> Because of + the fact that + clause = Because of + noun/ V-ing = Because + clause: bởi vì
23. D. Although he was very tired, he kept on working.
=> Although + clause: mặc dù
24. C. We liked the trip very much even though it was boring.
=> Even though + clause: mặc dù
25. B. Because of the improvement of the medical care, we live healthier.
=> Because of + noun/ V-ing = Because + clause: bởi vì
26. C. Despite his richness, he feels unhappy.
=> Although + clause = Despite/ In spite of + noun/ V-ing
27. A. She accepted the job because of the high salary.
=> Because of + noun/ V-ing = Because + clause: bởi vì
28. D. B & C are correct.
=> Although + clause = Despite/ In spite of + noun/ V-ing = Despite/ In spite of + the fact that + clause: mặc dù
29. A. Because of its importance, people want to learn English.
=> Because of + noun/ V-ing = Because + clause: bởi vì
30. C. Although it was snowing, I felt warm.
=> Despite/ In spite of + the fact that + clause = Although + clause: mặc dù