Riddle for high IQ people only: Find the digit in the question mark

This puzzle with the given numbers: 9, 4, 2, 6, 7, 3, 5, 1 has connected lines. Your task is to find the number in the question mark so that it fits and follows the rules.

This puzzle is only for people with high IQ, can you find the digit in the question mark? To find the answer in 10 seconds, your intelligence is definitely not ordinary.

To find the answer as quickly as possible, calmly reason and find the hidden rules of the numbers and the lines connecting them. Only when you find the rules of the numbers will you have the most accurate solution. This puzzle is really not simple, requiring you to have knowledge, reasoning skills and practical experience.

Scroll down to the comments section to write your answer, see if you and many others have the same exact answer.