Ranked by: number of candidates
1.
Comparison (223330 candidates)
2.
Present simple (153836 candidates)
3.
House and vocabulary (66551 candidates)
4.
Animals & insects (64155 candidates)
5.
Present simple (57666 candidates)
6.
Question tags (53059 candidates) 
7.
Passive voice (41315 candidates) 
8.
In the kitchen (41002 candidates)
9.
In the classroom - Vocabulary (40910 candidates)
10.
Jobs (40837 candidates)
11.
Superlative of superiority (34700 candidates)
12.
Pronunciation -ed in past simple (32525 candidates)
13.
Investigation (30024 candidates) 
14.
Means of transportation (29751 candidates)
15.
Idiomatic expressions (27889 candidates) 
16.
Sports- Vocabulary (27564 candidates)
17.
Superlative and the least (20901 candidates)
18.
Past continuous (19020 candidates)
19.
Been or Gone (17867 candidates)
20.
Idiomatic expressions (17159 candidates) 
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