Hydrogen cyanide (hcn) is a product of combustion that:
select one:
a. is released when wood and other natural materials are burned.
b. is equally as toxic as carbon monoxide (co).
c. targets the brain after being inhaled, ingested, or absorbed into the body.
d. acts as a respiratory stimulant, increasing respiratory rate and increasing oxygen to blood cells.

the entire statement is true and correct.  

there's no diagram, nothing is illustrated, and no question is asked.

the part of the wave he is travelling on is the crest

d) physical means can be used to separate the components of air.

speed of light in vacuum is 300,000 km/s

speed of light in other mediums is given by the expression v = c/n, where c is the speed of light in vacuum and n is the refractive index of medium.

for the first medium v = 200,000 km/s

  so, refractive index of medium = 300000/200000 = 1.5

    1.5 refractive index is usually for glass. so first medium is glass.

for the second medium v = 250,00 km/s

so, refractive index of medium = 300000/250000 = 1.2

    1.2 refractive index is usually for water. so second medium is water.

for the third medium v = 0 km/s

so, refractive index of medium = 300000/0 = ∞

    ∞ refractive index is usually for opaque mediums. so third medium is opaque medium.