a. Cannabis: This combination would likely dull the effects of the cocaine high because it (cannabis) helps persons relax. 

b. Heroin: This is what is referred to as a speedball. Though some people enjoy taking this combination, it is very dangerous and can lead to death. Cocaine, being a stimulant causes the heart rate to speed up. Heroin, a depressant, causes it to slow down. The heart becomes out of balance and the user can die.
One very famous person who died this way was John Belushi. He did a speedball which turned out to be lethal.

c. LSD: This is also another bad combination. A person that is hallucinating and paranoid with lots of energy could be dangerous to themselves and to everyone around them.

d. Ketamine: This combination is known as CK1, is very dangerous and should be avoided. f. Alcohol: Many people don’t know this, but this combination causes the liver to manufacture cocaethylene. This increases the risk of sudden death. Taking these drugs together can also be unwise because the effects of each, along with their risks, are intensified.