What will come in place of (−)? 1, 4, 3, 8, 7, 12, −, 16, 21

How many pairs of consecutive numbers are there in the following number sequence each of which is in ascending order (Like 1, 2)?
7 6 7 8 6 7 5 6 8 7 7 8 6 5 4 5 6 3 4

Select the number from among the given options that can replace the question mark (?) in the following series.
36, 75, 147, 297, 591, ?

In the following question, select the missing number from the given series.
4, 15, 2, 17, ?, 19