Which of the underlined words in the following paragraph is a transition word?

A National Park Service employee annually inspects the famous Mount Rushmore National Memorial near Keystone, South Dakota. He uses ropes and harnesses to take a close look at the 60-foot granite heads of George Washington, Theodore Roosevelt, Thomas Jefferson, and Abraham Lincoln. If he finds a crack, he coats it with a sealant, thereby preventing moisture from cracking it further.
a

annually
b

near
c

and
d

thereby