The formula for the half - life of continuosly decaying substances is M(t) = No h , where No is the initial amount, M(t) is the amount at time t, and h is the half - life of the substance. Now, given that a certain radioactive material of 160 grams has a half - life of 30 days. How much of the material would be left after 120 days? A 2.5 grams B 10 grams C 1 gram D 5 grams