If the first and sixth letters of the word $\text{"CREDENTIALS"}$, were interchanged, also the second and seventh letters and so on, which of the following would be the $8\text{th}$ letter from your right? 

From the word 'ASTOUNDER', what is the maximum number of independent meaningful words that can be made without changing the order of the letters and using each letter only once?

What is the missing letter?

Give the letter in place of the question mark(?).

$A, B, D, ?, K$