A skateboarder pushes the ground with one foot, and moves forward. Which statement BEST explains his motion?
The mass of the skateboard decreased causing acceleration to move him forward.
The unbalanced force exerted by the skateboarder’s foot caused him to move forward.
The mass of the student and skateboard increased causing acceleration to move him forward.
The balanced forces on the skateboard allowed it to change his motion and move him forward.

C or D i would think would be the answer

there is no correct list among the choices you've provided.

the list might look something like this:

-- baby human

-- piccolo

-- flute

-- soprano singer

-- trumpet

-- tenor singer

-- trombone

-- baritone singer

-- baritone horn

-- bass singer

-- tuba

since i had to make up the list myself, i made it easy on myself, and i didn't try to shuffle the string instruments and woodwinds into it too.

causes

fossil fuels burn

cities become more industrialized

human population grows worldwide