The impact of a sufficiently large asteroid or comet is one of the theories that have been put forward to explain why dinosaurs, as well as many other plant and animal species, became extinct 65 million years ago. why would this extinction-level event have had such a major impact on life on earth?
a. the toxic fumes produced meant that the dinosaurs could no longer breathe.
b. it increased the temperature of the planet.
c. it created enormous tidal waves and huge forest fires.
d. it produced dust and other particulates that would block out the sun.

d. It produced dust and other particulates that would block out the sun.

Explanation:

There is a theory that 65 million years ago, a rock 10 km in diameter from space impacted what is now the Yucatan Peninsula in Mexico. The brutal collision led to a series of events that caused one of the worst episodes of extinction of all time. More than three quarters of all the species that populated then our planet disappeared completely. Among them, the powerful non-avian dinosaurs.

The meteorite caused earthquakes, tsunamis and even volcanic eruptions. But in addition, scientists believe that the force of the impact threw a colossal amount of vaporized rock into the atmosphere, where it condensed into small particles known as spherules. As the spherules fell again, they were heated by friction at temperatures high enough to cause global fires and raze the planet's surface. A thin layer of spherules can be found worldwide in the geological record.

Well, those global fires threw large amounts of soot into the air that, according to new work published in the Proceedings of the National Academy of Sciences (PNAS), plunged the Earth into darkness for almost two years. This prevented plants from performing photosynthesis and dramatically cooled the planet.

showing a photograph would make this a hell of alot easier

"1. it should be, it has been for about 20 years, and it's really not much different in the end than breeding animals to have particular traits. there are always concerns about genetically engineered animals/plants affecting the environment in unexpected negative ways, but i can't think of an example where the fears didn't turn out to be baseless.  

first, as you point out, it would be a great advance in pharmacology, to be able to tailor drugs and their doses to me. the pros involve better medical treatment. the cons involve privacy issues like insurance companies denying coverage to people with genetic risk factors. looking at dna markers wouldn't affect the frequency of genetic diseases, but it would allow detection of them earlier. and ok, actually i suppose that if people used this information to avoid having children with genetic diseases, some might decrease in frequency."