be busy doing sth Grammatically speaking,what does 'doing' work as in this phrase？

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Grammar Challenge... Which sentence is grammatically correct? A) She don’t like coffee. B) They doesn’t understand the instructions. C) He doesn’t enjoy spicy food. D) We don’t has enough time.

Choose the correct answer.

English students, try this! Topic: Antonyms

The question about pizzas, the key says c but I think B is OK. I don't know which one is correct.

This began to change in the 1840s,among the pool factors were the young country's industrialization and western expansion. Here "the pool factors",what does that mean?

be busy doing sth Grammatically speaking,what does 'doing' work as in this phrase？

How many letters are there in the Arabic alphabet, which ones are considered the most difficult to pronounce or learn, and which Arabic letters have similar sounds to those in the English alphabet?"