Test 4, section 4, question #27. Why is the answer D instead of C? Please explain this.

The following expression represents the concentration, in microbubbles per milliliter, of the microbubble suspension remaining in a client t seconds after the technician has begun the ultrasound process.

100,000,000 ⋅ 0.5^(t/120)

Which expression shows the multiple of the concentration remaining each minute as a constant or coefficient?