The federal government expanded the national parks system due to the financial benefit of setting aside land assets to borrow against.
the need to use federal land for the expansion of the transcontinental rail system.
the activism of environmental groups like the Sierra Club and the National Audubon Society.
the lobbying of urban progressive groups to preserve endangered wildlife on protected land.

2/B - the need to use federal land for the expansion of the transcontinental rail system.

Explanation:

The 1966 Act required that all historical parks be entered in the National Register of Historic Places. Along with the Wild and Scenic Rivers Act, this legislation expanded the diversity of units in the National Park System.

c) troops march forward in close formation against newer, more accurate weapons

increase in population