Physics

Sound waves cannot travel through vacuum.

A microwave is the fastest communication channel whereas optical fiber is a way of transferring data not a way of communication.

Energy per unit volume is dimensionally equal to pressure.

Boiling point of heavy water is lower than that or ordinary water is not a property of heavy water.

Gravity is pulling the man down while friction with the air slows him down. With the parachute out it adds more friction slowing him down because air resistance works against the very large surface area of the parachute. A free falling skydiver that has not yet opened his parachute can make his downward speed vary between about 110 mph and 225 mph using air resistance. A spread eagled position presents the maximum area, and we fall the slowest.