We say: "lone wolf", "recluse", "antisocial", "withdrawn", "standoffish" , "introverted".

A reflector is not a mirror, it's a little plastic thing that is often put on streets or on bikes that reflect light. These are reflectors: http://www.toolsandleisure.co.uk/ekmps/shops/toolsandleisure/images/stick_on_reflectors_%28600_x_345%29.jpg We would never say "reflecting mirror" it sounds strange. I hope this helps! :)

There is no difference between the two sentences, but "why can I not" is much more formal and not as commonly used. To distinguish "can" and "can't" when they're pronounced, listen for the length of the word. "Can't" should have a hard "T" sound at the end, but not all English speakers pronounce the "T" as clearly, which makes it sound very similar to "can", but the "a" and the "n" sound is usually held longer. Another trick is that most people pronounce "can't" with a more pronounced 3rd tone (even though there are technically no tones in English, ha ha). "Can't" sometimes sounds like "cand" as well. "Can" usually sounds like a 2nd or 4th tone unless in a question like, "I can?" when it also sounds like a 3rd tone. But since English is not truly tonal, those are just guidelines that might help sometimes. If all else fails, just ask, "Did you say you can or you can't?" Even English speakers sometimes have to ask this when people are not pronouncing clearly, it won't come across as strange. :)

No, there's no difference at all. :)

A benchmark usually means a standard, but in this sentence it basically means compare. The best members are the benchmark or standard, so to benchmark against them is to compare yourself against them. Also, "compare against" doesn't mean anything different from "compare". So the sentence essentially means: participants can compare themselves to the best members to determine how good (or bad) they are.