Which of these components is not necessary for current to flow in a series or parallel circuit?
a. wire
b. switch
c. resistor
d. all of these are needed.

All the components (D) ie, wire, switch and resistor are needed for current to flow in a series or parallel circuit.  
In a simple series circuit, all the components are connected end-to-end to form a single path for the electrons or electric current to flow through the circuit. A simple parallel circuit is where all the components in the circuit are connected between the same two sets of common points, creating a more than one path for the electrons or electric current to flow around the circuit. 

Only wire is necessary.

A switch is only needed if you want to be able to turn the current on and off easily.

A resistor is only needed if you want to generate some heat or limit the current.

from 21 km/h to 31 km/h

Explanation: The more speed it has the more kinetic energy it is required for it to go faster.