Answer: here.
Explanation:
The correct is C) the transcontinental railroad
The open door policy was principles that was installed in 1899 and then the 1900s to Have a protection of equal or even trading among countries
The Mali Empire collapsed in the 1460s CE following civil wars, the opening up of trade routes elsewhere, and the rise of the neighbouring Songhai Empire, but it did continue to control a small part of the western empire into the 17th century
(Hope this helps)