Given a scientific passage and an answer, generate a question for the given answer.

Ex Input:
Passage: You can't go to the mantle but small pieces of the mantle can come to you. Kimberlite forms deep in the mantle and is erupted violently into the crust. Kimberlite can contain diamonds. Most kimberlites reached the surface much earlier in Earth history. Answer: crust

Ex Output:
What is the upper most level of earth's layers called?

Ex Input:
Passage: Kevin Walsh. Cows are able to digest grass with the help of the bacteria methanogens in their gut . CC BY 2.0. Answer: bacteria

Ex Output:
A type of what in a cow's gut enables the animal to digest grass?

Ex Input:
Passage: Amphibians are vertebrates that exist in two worlds. They divide their time between freshwater and terrestrial habitats. They share a number of features with air-breathing lungfish, but they also differ from lungfish in many ways. One way they differ is their appendages. Amphibians are the first true tetrapods, or vertebrates with four limbs. Modern amphibians include frogs, salamanders, and caecilians, as shown Figure below . Answer: tetrapods

Ex Output:
Vertebrates with four limbs are also called what?