There will be a blackout, unless the heatwave abates. This is saying that if the heatwave continues, there will be a blackout.

If the heat wave does not abate, then it will cause a blackout.

If no blackout occurs, then the heat wave must have abated. Otherwise, the heat wave would have caused a blackout.

If the heat wave abates, there are still other threats that could cause a blackout, such as unrelated equipment failure.

Couldn't each of these hypotheses be contributing to the observed phenomenon? Thus, both are potential links in the causal chain. They're not necessarily mutually exclusive and therefore not necessarily competing. Toxic runoff and algal bloom could both be contributing to dolphin deaths.

Some heavily implies more than one. I don't know a single context where you'd use "some" to describe only 1 of something.

#feedback Question 1 could be confusing to some. The fact that some mammals lack limbs does not technically negate the first statement that all non-water-breathing mammals have limbs. The mammals without limbs could all breathe water, allowing the first statement to remain true. The first statement does not establish whether or not water-breathing mammals exist. It requires outside knowledge that no mammals actually breathe water, which is a common misconception about marine mammals. Right now, Q1 is logically equivalent to: All Claussen's pickles are kosher is negated by some pickles are not kosher. For the second statement to negate the first, it must be established that Claussen's are the only pickles to exist.