What will come in place of the question mark (?) in the following number series? 1, 3, 6, 10, 15, ? MCQ with Answer and Explanation

The differences between consecutive terms are: 3-1=2, 6-3=3, 10-6=4, 15-10=5. The differences are increasing by 1 each time. The next difference should be 6. Therefore, the next term is 15+6=21.[reference:1]