A sentence In this sentence:"This increase may be due as much to advances in surveying techniques, which simply enable counters to find more manatees, as it is to an actual rise in the population.", what does the word "it" refer to? and please explain the usage of "as" in this sentence. Thaks

what's the difference between "refrain" and "forbear" ? Here is a sentence: In spite of all his efforts, he could not ___ from uttering a deep groan, which, however, was lost amid the noisy felicitations of the company. According to the answer list, "refrain" is chosen for the blank. But I don't know why can't use "forbear" in this sentence, because I find the definitions of the two words similar. Please help me explaining the difference between the two words and it's better if you give me some examples of them. Thank you.