Mountain biking is a subset of cycling, like off-roading with a jeep is another type of car driving. From a riders perspective (XC) mountain biking is more dynamic, more out-of-the-saddle work with bigger changes in pace. Road biking is more about endurance and slogging it out mile after mile. Then there are the subsets of MTBing, like downhill, which is a phenomena all to itself.
you could, because they are essentially the same. But it would be very hot because of all the padding. But I've seen helmets where the padding just comes out and it can be used as a biking helmet like Helt-Pro. You can buy one helmet with winter hat, and some changeable summer hats and use it for biking)
== == It is used at the end of a sentence to describe that something is in the extreme level or in a higher level than others. In the sentence, "I like hiking, swimming, and running but I like biking the most." Biking is the thing which is in the extreme position as it is liked the most. It is liked more than hiking, swimming and running.