If $n$ is even and the value of $^nC_r$ is maximum,then $r = $

In how many ways can $6$ persons stand in a queue at a time?

How many numbers lying between $10$ and $1000$ can be formed from the digits $1, 2, 3, 4, 5, 6, 7, 8, 9$ (repetition is allowed)?

The Chief Ministers of $11$ States of India meet to discuss the language problem. In how many ways can they seat themselves at a round table so that the Punjab and Madras Chief Ministers sit together?

The number of four-letter words that can be formed using the letters of the word $BARRACK$ is

The number of $4$-digit even numbers that can be formed using $0, 1, 2, 3, 4, 5, 6$ without repetition is