TASK DEFINITION: In this task you are given a sentence. You must judge whether subject of the main clause is singular or plural. Label the instances as "Singular" or "Plural" based on your judgment.
PROBLEM: The man-made chemical polonium leaks from the engines by tiny droplets.

SOLUTION: Singular

PROBLEM: " Those aliens attacked us, and earth was destroyed because of what they did.

SOLUTION: Plural

PROBLEM: The Italian customs guards believed themselves obliged to notify the American Military Police to come look at the U.S. travel authorization documents, which took time.

SOLUTION:
Plural