a bit string is a finite sequence of 0's and 1's. how many bit strings have length 7? bit strings (b) how many bit strings of length 7 begin with three 0's? if a bit string of length 7 begins with three 0's, then positions remain to be filled in. since each of these positions can be filled with either a 0 or a 1, the number of bit string of length 7 that begin with three 0's is . (c) how many bit strings of length 7 begin and end with a 1? bit strings