If it is possible to make a meaningful word with the fourth, the eighth and the tenth letters of the word 'COUNTERACT', which of the following will be the last letter of that word ? If no such word can be made, give X as the answer. If more than one such word can be made, give M as the answer.
Number of letters skipped in between the adjacent letters in the series are consecutive even numbers. Which of the following series observes this rule ?