Read the following context and choose the best option to answer the question.
Context: The next thing I knew we were at the Big Dam Bridge . Walking up it is quite a bit easier than running or cycling up it , I assure you . Ha !
Question: Why is it easier to walk up the Big Dam Bridge ?
Options: 
A. It takes less mental effort .
B. It takes much longer .
C. It is much faster .
D. It takes less physical effort .


D