The correct answer is: They show that the money for the project came from investors who believed in it.
This supports Low's opinion that the bridge is "a triumph of faith" because it highlights that the financial backing for the bridge relied on the trust and belief of investors in the project's success and in the ability of the two cities to repay the loan. The faith of these investors was crucial for the bridge's construction, demonstrating the important role of belief in the realization of such a monumental project.