To start the engine, you need more than just gas. you need ​ (a)   for it to burn.

• air
• exhaust
• fuel
• explosion
• combustion
• ignition
• spark

The ​ (a)   in the engine move up and down.

• pistons
• carburetor
• fuel line
• spark plug
• radiator
• battery

The ​ (a)   creates a ​ (b)   to start burning the gas.

• spark plug
• spark
• carburetor
• pistons
• explosion
• ignition
• radiator

When the air, gas, and spark combine, it causes a controlled ​ (a)   .

• explosion
• spark
• ignition
• combustion
• exhaust

Just like when other things burn, the burning gas creates smoke. The smoke travels through the ​ (a)   and out the back of the bus.

• exhaust pipe
• radiator
• pistons
• carburetor
• fuel tank