A shell is fired from a fixed artillery gun with an initial speed u such that it hits the target on the ground at a distance R from it. If t1 and t2 are the values of the time taken by it to hit the target in two possible ways, the product t1t2 is : [Main 12 April 2019 I]
(a) R/4g
(b) R/g
(c) R/2g
(d) 2R/g