A hot-air balloon travelling vertically is subjected to three forces: its weight, the force from air resistance, and the upthrust force. The hot-air balloon is descending vertically at a constant speed. The air resistance force acting on the balloon is $F$. What weight of material must be released from the balloon so that it rises vertically at the same constant speed?
- A$F$
- B$2F$
- C$3F$
- D$4F$