The answer is the first option: I, II and III.
The explanation is shown below:
1. By definition, there is a joint variation when a variable depends on two or more different variables. Therefore, you can express it as following:
Where and
are the variables and
is the constant of proportionality.
As you can see, is directly proportional to
and
.
2. Keeping the information above, you have:
I) (
varies jointly with
,
and
.
II) (If
is the constant of proportionality,
varies jointly with
and
).
III) (
varies jointly with
and
.