The scientists  the causes of cancer are making progress. A. research

B. researched

C. researching

D. researches

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1. past simple, past perfect
2. past simple, future in the past
3. past simple, past simple
4. past simple, past simple
5. past simple, past perfect
6. past simple, past simple
7. past simple, past perfect
8. past simple, future in the past
9. past simple, future in the past
10. past simple, past simple
11. past simple, past simple
12. past simple, past perfect
13. past simple, past perfect
14. past simple, future in the past
15. past simple, future in the past
16. past simple, past perfect
17. past simple, future in the past
18. past simple, future in the past
19. past simple, past continuous
20. past simple, past perfect continuous

2) the red dog big ≠the black dog
The red dog is not as/so big as the black dog.
3) the chair high =the bench
The chair is as high as the bench.
4) the girl tall =the boy
The girl is as tall as the boy.
5) the green tree tall ≠ the yellow tree
The green tree is not as tall as the yellow tree.
6) the sitting room cosy ≠ the bedroom
The sitting room is not as cosy as the bedroom.
7) the kitchen clean = the hall
The kitchen is as clean as the hall.
8) the garden green = the forest
The garden is as green as the forest.