C.
Thanks me later, that's my answer.
"Reflection" is the change direction of waves when blocked by obstacle
¡Hello!
Carbon monoxide is an odorless, colorless and tasteless gas produced by the incomplete combustion of carbon in fossil fuels such as wood, propane, charcoal, oil, gas, coal or other fuel.
Density is mass divided by volume. rho=m/v. So, v=m/rho. In frank's case this is 80/8 = 10 cm^3.
Using the given equation:
di = 20.0 * 10.0 / 20.0 - 10.0
di = 200/10
di = 20.0 cm
The answer is A.