Why have to be every dog has its day?

[golf]

Hi Alan, 8)

Please help me again. for this sentence I don't work out why have to be just dog. why dog is the right correct?
Test No. incompl/inter-38 "Save the Day", question 6

Don't worry your bad luck will soon end. Remember every ......... has its day.

(a) dog
(b) cat
(c) mouse
(d) rat

Test No. incompl/inter-38 "Save the Day", answer 6

Don't worry your bad luck will soon end. Remember every dog has its day.

Correct answer: (a) dog

Your answer was: incorrect
Don't worry your bad luck will soon end. Remember every rat has its day.

Thank you for your advance

[Alan]

Hi,

For some reason 'dog' is often used to represent someone who is having a bad time. You can see that in expressions like 'lead a dog's life'- have a miserable time, 'be in the doghouse' - be in trouble. So in this expression: 'Every dog ....' it means even the most unfortunate person has some luck some time.
Alan
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