7. A 100-gram radioactive substance has a half-life which is 15 years. A half-life of 15 years means that half of the amount of the substance will be left
after 15 years. How many grams of the substance
will be left after 60 years?
A. 6.25 grams
C. 25 grams
B. 12.5 grams
D. 50 grams​