Answer:
San Francisco, a city in California, has many hills.
Step-by-step explanation:
An appositive is a noun or noun phrase that comes before or after another noun and identifies or explains it. Just like in this case, the appositive usually comes after the noun it refers to and is framed by commas.
The second sentence gives us additional information about San Francisco. In other words, it explains what San Francisco is. This is why we can use it to expand the first sentence by turning it into an appositive. That is how we'll get the following sentence:
- San Francisco, a city in California, has many hills.