If the area bounded between the X-axis and the graph of y=6x−3x2 between the ordinates x=1 and x=a is 19 sq units, then 'a' can take the value
a. 4 or −2
b. two values are in (2,3) and one in (−1,0)
c. two values one in (3,4) and one in (−2,−1)
d. None of the above