Question 1. Is the effect of drag force on the bike and rider greater at slow or high speeds?

Answer: Greater at high speeds
The fact that at higher speeds the aerodynamic drag of the bicycle consumes nearly all the rider's effort was realised very early in the bicycle's history. As early as 1895 disk wheels were offered for bicycles to reduce aerodynamic drag.
The Exploratorium website has an interesting section related to this question.
see: http://www.exploratorium.edu/cycling/aerodynamics1.html

Question 2.What is a derailleur?

Answer: A complex gear-changing mechanism that moves the bicycle chain smoothly from one gear to another. The gears are different sizes and the chain has to be long enough to be moved onto the large gears so the derailleur mechanism keeps a constant tension on the chain to prevent it drooping loosely when the smaller diameter gears are engaged. The derailleur is an important but sometimes difficult part of the bicycle to keep in good working order. (The gears of the derailleur are also called cogs or sprockets.)

Question 3. Bicycle designers work hard to make their bikes aerodynamically efficient. What strategies can a rider use to be more aerodynamic?

Answer: You will notice in the image used to illustrate this stumper that one rider is sitting fairly upright on his bike while the other is crouched over the handlebars in order to provide a smaller and more streamlined (aerodynamic) frontal area. This rider is wearing a helmet, which is both protective and streamlined. His clothes are more tight fitting and so do not flap in the wind. The rider sitting upright could easily be more streamlined by leaning forward even though his bike is shaped less aerodynamically. Very serious bike racers will often shave their legs to further reduce the drag force.

The following list shows some ways of reducing aerodynamic drag:

Streamlined, well-fitted helmet

Clipless pedals.

No water bottle and cage

Tape over shoe laces

Pump under top tube instead of in front of seat tube

Smooth nylon socks

Smooth shoe covers

One-piece skin suit with short sleeves and legs