for the first question you can put in some numbers...
suppose the truck without sand is traveling a 6 m/s and weighs 10 kg... its momentum is now 60 units
2 kg of sand is added... the momentum of the full truck is still 60 units but the mass is 12 kg so the speed is now 5 m/s
as the 2kg of sand drops out it is moving at 5 m/s so takes 10 units of momentum with it... so 50 units are left for the truck... 10 x ? = 50.... ? = 5 m/s so the truck does not slow down or speed up.
answer = B