Here we apply conservation of linear momentum. The momentum of the truck with cargo and without cargo remains constant. That is,
.
Here 
are initial mass and velocity. 
are final mass and velocity. Here 
and 
.
The velocity of the truck be after its cargo is taken off is
The lens changes the focal distance of the eye so that you can focus on images/objects.
I think it is B, because the sun’s size is pretty average
