This is because if the price of such good rises, there is no good the consumers can switch to as a substitute so the demand of such good is inelastic/elastic. For example cooking salt will have an inelastic/elastic demand if the price of cooking salt increases, because there is no exact substitute good for a cooking salt.
Part D stated any number can be put and the equation will still be the same. I already answered this question for another guy and tou can go search for that if you want to see edmentums sample answer that states they all work.
Well, 9x5=45 and if you calculate that in cubic meters i believe it would be 243m³ (I'm in 7th grade...=/) HAPPY EARLY VALENTINES DAY! HOPE YOU GET THIS RIGHT! =D