What are two ways that natural bridges form?

    Natural bridge, also called natural arch, naturally created arch formation resembling a bridge. Most natural bridges are erosion features that occur in massive, horizontally bedded sandstone or limestoneMay  be produced by entrenched rivers eroding through meander necks to form cutoffs. Still others are produced by exfoliation and may be enlarged by wind erosion. 

Ithink the answer to that would be b):

I'm pretty sure it's c. extremely gradual but not 100% sure though.