The sentence contains:

Thrilled by the discovery of gold at Sutter’s Mill, thousands of prospectors rushed to California, traveling by boat, by covered wagon, and walking.]]> Which item in the list is constructed differently from the other two?

Thrilled by the discovery of gold at Sutter’s Mill, thousands of prospectors rushed to California, traveling by boat, by covered wagon, and walking.]]> Thrilled by the discovery of gold at Sutter's Mill, thousands of prospectors rushed to California, traveling by boat, by covered wagon, and by walking. Rewrite the list item "walking" to match the construction of the other list items. Then submit it and say the sentence.

Thrilled by the discovery of gold at Sutter’s Mill, thousands of prospectors rushed to California, traveling by boat, by covered wagon, and walking.]]>