In a vertical U-tube containing a liquid, the two arms are maintained at different temperatures t1 and t2. The liquid columns in two arms have heights l1 and l2 respectively. The coefficient of volume expansion of liquid is equal to............
a. l1 - l2 /l2t1 - l1t2
b. l1 - l2 /l1t1 - l2t2
c. l1 + l2 /l2t1 + l1t2
d. l1 + l2 /l1t1 + l2t2