Difference between 'turn in' and 'turn down'

[LOMPO]

Test No. incompl/inter-33 "Story (2)", question 5

Most of the residents were elderly and so they all invariably turned ......... early most evenings.

(a) down
(b) over
(c) in
(d) into

Test No. incompl/inter-33 "Story (2)", answer 5

Most of the residents were elderly and so they all invariably turned in early most evenings.

Correct answer: (c) in

Your answer was: incorrect
Most of the residents were elderly and so they all invariably turned down early most evenings.
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GOOD MORNING.

MY QUESTION IS ABOUT THE USE OF "TO TURN' PLEASE SEND ME MORE EXAMPLE AND EXPLANATION ON IT.

[Alan]

Turn in is an expression meaning go to bed/go to sleep.
Turn down is an expression meaning reject/not accept as in turn down an invitation meaning to say you won't accept the invitation.
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English as a Second Language
You can read my ESL story Passive Voice