Answer: Because the acre is a measure of area, it has no definite perimeter. For example, using the examples in question one, the perimeter of the long, skinny sidewalk acre would be 17,434 feet (more than 3 miles), but the perimeter of the square acre would be 835 feet.
No other molecular fluoride of krypton has been isolated, so all krypton compounds are derived from KrF 2, where Kr is in the +2 oxidation state. Krypton difluoride is a powerful oxidative fluorinating agent. (Its oxidizing power means that it extracts electrons from other substances and confers on them a positive charge.
Superman (and, Kryptonians) has superpowers on Earth because of our yellow Sun (Krypton's Sun was red). Kryptonite blocks that ability. Also, its radiation is poisonous to Kryptonians which can kill them (although, I have read comics in which Superman had developed some immunity against green Kryptonite).