a. Ancient: as an adjective, it relates to a remote period of time. As a noun, it refers to an aged person or a person that lived in ancient times. Here's a sentence with this word:
“Ancient Greece history is long, yet very interesting.”
b. Limits: a limit is “a point or level beyond which something does not or may not extend or pass.”
“Thanks to the system of Checks and Balances, all three branches of the government has limits to their power”
c. Obviously is an adverb meaning “clearly.”
“I was obviously uncomfortable with all that noise”