Please help! 8th grade math 50 points

Me with this one question, and i'll upvote the brainliest answer

How to find the exponential function y=ca^x, with points (1,2) and (2,1)

The weight of a radioactive isotope was 96 grams at the start of an experiment. after one hour, the weight of the isotope was half of its initial weight. after two hours, the weight of the isotope was half of its weight the previous hour. if this pattern continues, which of the following graphs represents the weight of the radioactive isotope over time? a. graph w b. graph x c. graph y d. graph z