Question: You would like to know how many different categories you have. Which query will best get the job done?

1. `db.vehicle.distinct("category")`
2. `db.vehicle.unique("category")`
3. `db.vehicle.distinct("category").count()`
4. `db.vehicle.distinct("category").length`

Answer: The correct answer of the above question is Option D:`db.vehicle.distinct("category").length`