Why bottom instead end?

[Thameem]

English Language Proficiency Tests, Advanced Level

ESL/EFL Test #99 "The Parliamentary Candidate (2)", question 5

At half past eight Henry was in conference with his election agent, Andrew Higgins, at party headquarters.
Andrew: If you want my opinion, I suggest you cancel all your engagements for today and wait until the police get to the ......... of the matter.

(a) base
(b) end
(c) bottom
(d) hind

English Language Proficiency Tests, Advanced Level

ESL/EFL Test #99 "The Parliamentary Candidate (2)", answer 5

At half past eight Henry was in conference with his election agent, Andrew Higgins, at party headquarters.
Andrew: If you want my opinion, I suggest you cancel all your engagements for today and wait until the police get to the bottom of the matter.

Correct answer: (c) bottom

Your answer was: incorrect
At half past eight Henry was in conference with his election agent, Andrew Higgins, at party headquarters.
Andrew: If you want my opinion, I suggest you cancel all your engagements for today and wait until the police get to the end of the matter.
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why bottom instead end? Please explain what's the differnce between bottom and end? under what situation we use bottom or end?

[Mister Micawber]

'The bottom of the matter' is an idiom. It means 'the solution of the matter'.
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Native English teacher at Mister Micawber's