The half life of a certain substance is about 4 hours. the graph shows the decay of a 50 gram sample of the substance that is measured every hour for 9 hours.

which function can be used to determine the approximate number of grams of the sample remaining after t hours?

a
y = 50(0.85)x

b
y = 25(0.15)x

c
y = 50(0.15)x

d
y = 25(0.85)x