Select the gas.
A: marbles
B: air inside a beach ball
C: pipe cleaner
D: baseball cap
A baseball cap is a solid. A solid has a size and shape of its own. If you put a baseball cap on your head, the baseball cap will still have a size and shape of its own.
A pipe cleaner is a solid. You can easily bend a pipe cleaner. But it will still have a size and shape of its own.
A marble is a solid. A solid has a size and shape of its own. A marble can be made of clear or colored glass.
The air inside a beach ball is a gas. A gas expands to fill a space. The air fills all the space inside the beach ball. If air leaks out, it will expand into the space around the ball.