A high school student babysits to earn money to buy a DvD pl

A high school student babysits to earn money to buy a DvD player.What is the independent variable of this function?
A:the money he earns
B:the cost ofthe DVD player
C:the time thst he spends babysitting
D:the size of the house
First you need to research the difference between an independent variable and a dependent variable in mathematics.
2.Now if you read this problem over and over again,ask yourself this question:
A student tries to earn money to buy something he likes (DVD player).
So we have a student,money,DVD player and don't forget,the time he spends babysitting to earn money.
3.Of all the items listed,which is really an independent variable.This means it happens without being
controlled by outside factors.
4.Yes,the answer is C:the time that he spends babysitting.But can you explain why this is the only possible
answer?
还有附加题;ADDITIONAL MATH PRACTICE PROBLEMS:
a.Solve for X:2X - 5 = -25
b.Solve for X:-9X - 2X + X = 50
c.write the reciprocal of 12
d.Is -Y + 12 = 20 a step used in solving for Y in 7Y - 4(2Y -3) = 20 ; Yes or No?
e.Is 2X = -5 a step used in solving for X in -3X + 12 = -5X + 7 ; Yes or No?
f.Does the number Zero have a multiplicative inverse value; Yes or No?
g.In 9 - ( 8Q + 6 ) = 10Q - 8 ; is 11 = 18Q is step in finding Q ; Yes or No?

4.This is the only answer because the cost of DVD is a constant and does not change,the money he earns depends on the hours he worked,so the only varaible that matters is the hour he works.
a.-10
b.-5
c.1/12
d.Yes
e.Yes
f.No
g.Yes