The elevation of Lake Ontario is $75.00 \mathrm{~m}$ and the elevation of Lake Erie is $174.28 \mathrm{~m}$. A ship travels between the two lakes, passing through the locks of the Welland Canal. If the ship takes 8 hours to travel between the lakes, the average (mean) change in elevation per hour is
(A) $12.41 \mathrm{~m}$
(B) $21.79 \mathrm{~m}$
(C) $5.25 \mathrm{~m}$
(D) $4.14 \mathrm{~m}$
(E) $7.80 \mathrm{~m}$
In travelling between the two lakes, the total change in elevation is $174.28-75.00=99.28 \mathrm{~m}$. Since this change occurs over 8 hours, the average change in elevation per hour is $\frac{99.28 \mathrm{~m}}{8 \mathrm{~h}}=12.41 \mathrm{~m} / \mathrm{h}$.

Answer: (A)