NDA 2021 Chemistry question

Chlorine occurs in nature in two isotopic forms of masses 35 u and 37 u in the ratio of 3 : 1 respectively. What is the average atomic mass of the Chlorine atom ?

• (A)36·1 u
• (B)35·5 uCorrect
• (C)36·5 u
• (D)35·1 u