Find the mistake in the four underlined parts A, B, C or D of each sentence

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Task 3. Find the mistake in the four underlined parts A, B, C or D of each sentence.
1. If someone came into the store, smile and say, "May I help you?"
A
B
D
2. If you try these cosmetics, you look five years younger.
A
B
с
D
3. If you do not understand what were written in the book, you could ask Mr. Pike.
A
B
с
D
4. I will come to meet Mr. Pike and tell him about your problems if you didn't solve
A
B
с
D
them yourself.
5. Sam will not graduate unless he doesn't pass all the tests.
A
B
C
D
6. If there isn't enough food, we couldn't continue our journey.
A
B
C
D
7. Unless you pour oil on water, it will float.
A
B
с
D
8. You have to take a taxi home if you want to leave now.
A
B
с
D
9. If anyone will phone, tell them I'll be back at 11:00.
A
B
с
D
10. We can hire a mini bus if there will be enough people.
A
B
C
D