Answer:
Its A! i THOUGHT IT WAS d AND I GOT IT WRONG ITS A!!
Explanation:
Singular noun is a noun that assigns to one thing, one person and one idea. Example: Boy, Girl, Father, Mother, Man, Doctor and so on.
I believe it is C. In writing, we can more easily conceal our attitudes.
Working out: 6.25% = 4 half lives (20x4=80ma) 80ma is how old the rock is