:3. A coin is tossed 100 times,  The difference "twice the
number of heads - the number of tails" is like the sum of 100
draws from one of the following boxes.  Which one? <p>

a.  A box with two 1's and one -1 <p>

b.  A box with three tickets, two of which are marked
         "three" and one of which is marked with a "tails" <p>

c.  A box with two 1's and a 0 <p>

d.  A box with one 2 and one -1 <p>