From the table, the average mass increases by $18$ grams each year: $138-120=18$, $156-138=18$, and $174-156=18$. So the data follow a linear pattern with a constant increase of $18$ grams per year. Continuing the pattern gives year 5 as $174+18=192$ and year 6 as $192+18=210$. Therefore, the statement that must be true is that the average mass in year 6 will be $210$ grams.

For a rectangle, area $=$ length $\times$ width. The graph represents all pairs $(x,y)$ such that

When the length is 15 feet, substitute $x=15$ into the equation:

So the width is closest to 6.4 feet.

The correct answer is $y=3x$. To find a model, compare the parking fees to the number of visitors in the table. For example, when $x=120$, the fees collected are $y=360$, and $\frac{360}{120}=3$. Check another row: when $x=150$, $\frac{450}{150}=3$. The same ratio appears in the other rows as well, so the amount collected is always 3 times the number of visitors. Therefore, the equation that models the relationship is $y=3x$.

From the table, when the month increases from 1 to 3, the number of subscribers increases from 420 to 500, which is an increase of 80 over 2 months. So the rate of change is $\frac{80}{2}=40$ subscribers per month. That means the linear expression has the form $40m+b$. Using the point $(1,420)$, substitute to find $b$: $420=40(1)+b$, so $b=380$. Therefore, the expression is $40m+380$.

First find the slope of the line using the two given points:

So the equation has the form $y=30x+b$. Substitute one of the points, such as $(4,180)$:

Therefore, $b=60$. The equation is

which matches choice A.