Atomic weight of Boron is 10.81 and it has two isotopes ₅B¹⁰ and ₅B¹¹. Then the ratio of ₅B¹⁰ : ₅B¹¹ in nature would be __
(a) 15 : 16
(b) 10 : 11
(c) 19 : 81
(d) 81 : 19