Which sentence is at the beginning of a paragraph?

1. the sentence that supports the main idea
2. the sentence that expands on the main idea
3. the topic sentence
4. the concluding sentence

The sentence needs a descriptor of exactly "what" will happen at gurley and marina. in this case, it is the light at the intersection turning green. in this case, the sentence could be rewritten as "the light at the intersection of gurley and montezuma will turn green exactly three seconds after the light at the intersection of gurley and marina."