What geographic feature protected Rome from an invasion from the north?

a. the Tiber River
b. the Italian Alps
c. the Hills of Rome
d. the Tyrrhenian Sea

in contrast, the biological controls are self-sustaining, inexpensive and very effective once established. biological control could be used on thousands of golf course acres, improving their biodiversity and quality as wildlife habitat. in the process, the golf industry will be shown to be leaders in an environmental stewardship program of benefit not only to the lands they manage but also to the watersheds in which they exist.

a and b!

dfroos means which is a and b