Three blocks with masses `m, 2m and 3m` are connected by strings, as shown in the figure. After an upward force `F` is applied on block `m`, the masses move upward at constant speed `v`. What is the net force on the block of mass `2m`? (Take, g is the acceleration due to gravity)
Options:
(a) Zero
(b) `2mg`
(c) `3mg`
(d) `6mg`