How to do the following quadratic word problem?

Question

The boiling point of water $T$ (measured in degrees) at altitude $a$ (measured in feet) is modeled by the function $T(a) = -.0018a + 212$. In terms of altitude and temperature, which statement describes the meaning of slope?

and the answer choices given are

1. The boiling point increases by 18 degrees as altitude increases by 1000 feet
2. The boiling point increases by 1.8 degrees as the altitude increases by 1000 feet
3. The boiling point decreases by 18 degrees as the altitude increases by 1000 feet
4. The boiling point decreases by 1.8 degrees as the altitude increases by 1000 feet

For this problem I don't even know where the 1000 came from and I am really confused. What does this question and the answer choices mean? Can anybody clear this up for me?

Answer 1

This is in the form $y=mx+b$, the standard slope-intercept form of the line.

Instead of $y$, we have $T$, the boiling point. Instead of $x$, we have altitude $a$.

The amount "$1000$ feet" is a typical change in altitude. It's not special beyond the fact that it's a convenient amount of altitude change for this problem.

The slope of this line is $-0.0018$. Since the left side ($T$) is in units of degrees Fahrenheit, and $a$ is in units of feet, the slope is in units of degrees Fahrenheit per foot.

The $212$ is the intercept, which is at altitude $0$ feet.

The part that depends on altitude is the $-0.0018a$. When you plug in $1000$ here, what is the change? This should give you the answer (choice (4)).