
Based on the function above, the lowest price that the promotion offers for a large pizza is 9 dollars. This is the cost of the pizza when $t=0$, $t=1$, $t=2$, and $t=3$. We know that $t$ denotes the number of days since February 10^{th}. Thus, $t=0$ corresponds to February 10^{th}, $t=1$ corresponds to February 11^{th}, $t=2$ corresponds to February 12^{th}, and $t=3$ corresponds to February 13^{th}. Therefore, the best days to give the new pizza place a try in order to get the best price are February 10, February 11, February 12 and February 13.

February 18^{th} is eight days after February 10^{th}, corresponding to $t=8$. Based on the function above, the cost of a large pizza in dollars is given by $C(t)=9+t$ when $3 \lt t \leq 8$. Then, when $t=8$, $C(t)=9+8=17$ dollars. Thus, the cost of a large pizza on February 18th is 17 dollars.

We know that $13 \neq 9$ which implies that a large pizza cannot cost 13 dollars when $0 \leq t \leq 3$ because for these values of $t$, $C(t) = 9$ based on the function above. Similarly, we know that $13 \neq 20$, which implies that a large pizza cannot cost 13 dollars when $8 \lt t \lt 28$ because $C(t)=20$ for these values. Thus, we know that the only time a large pizza could cost 13 dollars is when $t$ is in the interval $3 \lt t \leq 8$, for which $C(t)=9+t$. In order to find out which date a large pizza will cost 13 dollars we must plug 13 into this equation as our cost and solve for $t$:
$$
\begin{align}
13 &= 9 + t \\
t &= 13  9 \\
t &= 4
\end{align}
$$
This means that a large pizza costs 13 dollars when $t=4$. We know that $t$ denotes the days since February 10^{th} so $t=4$ corresponds to February 14^{th}. Thus, a large pizza will cost 13 dollars on February 14^{th}.

The statement that a pizza is at least $A$ dollars $B$ days into the promotion means that the cost of a large pizza $B$ days into the promotion, denoted $C(B)$, is greater than or equal to $A$ dollars. Thus, an expression that describes this sentence using function notation and mathematical symbols is simply
$$ C(B) \geq A $$

$C(9)=20$ and $C(8)=9+8=17$ based on the equation above. Thus, $ C(9)C(8)=2017=3$. $C(9)=20$ corresponds to the cost of a large pizza 9 days after February 10^{th}, or February 19^{th}. $C(8)=17$ corresponds to the cost of a large pizza 8 days after February 10^{th}, or February 18^{th}. Thus, the meaning of $C(9)C(8)=3$ in the context of the problem is that on February 19^{th}, a large pizza will be 3 dollars more expensive than a large pizza on February 18^{th}.

The first two weeks of the promotion take place from February 10^{th}, when $t=0$, to February 23^{rd}, when $t=13$. The average rate of change is given by:
$$
\frac{C(13)C(0)}{130} = \frac{209}{13} \approx 0.85
$$
The expression above shows that the cost of a large pizza goes up about 0.85 dollars, or 85 cents, per day during the first two weeks of the promotion period. Therefore, the expression that best describes this statement is:
$$
\frac{C(13)C(0)}{13}
$$