(1) One day, Lucy and Tom had a picnic. (2) Lucy spread a tablecloth in the grass beside the millpond it was a pretty day for a picnic. (3) All went well until Lucy discovered that ants had invaded the lemonade. (4) They had also overrun the potato salad.

Which number marks a run-on sentence—a group of words made up of two sentences?

2 Lucy spread a tablecloth in the grass beside the millpond.

Number 2 marks a run-on sentence, as it combines two separate statements in one sentence.

To identify the run-on sentence in the given passage, we need to determine which number marks a group of words made up of two sentences. Let's analyze each sentence:

(1) "One day, Lucy and Tom had a picnic." - This is a complete sentence.

(2) "Lucy spread a tablecloth in the grass beside the millpond. It was a pretty day for a picnic." - This is a group of two sentences.

(3) "All went well until Lucy discovered that ants had invaded the lemonade." - This is a complete sentence.

(4) "They had also overrun the potato salad." - This is a complete sentence.

After analyzing each sentence, we find that number (2) is the run-on sentence. It consists of two sentences merged together without appropriate punctuation or conjunction.