a hard nut to crack

The same question, asked 2 times: 14 Jul 2023 S3*, 27 Jul 2023 S1. Every wording below, oldest first.

Try each one first, then open its answer.

Question 1
14 Jul 2023 S3
90 $Select the appropriate idiom to fill in the blank. The police have not yet been able to solve the case; it seems to be ____________.$
Tap an option to check your answer.
Question 2
27 Jul 2023 S1
76 $Select the most appropriate meaning of the given idiom. Hard nut to crack$
Tap an option to check your answer.