Read the below given passage carefully and answer the question from the following given options:
A volcano is a burning mountain with a great hole running deep into the earth. Out of it come smoke, gases, and stones. The mouth of the opening is called the crater of the volcano.
Volcanoes are not always burning. Sometimes a volcano will remain quiet for centuries. And then it will suddenly become active. This is called volcanic eruption.
When the eruption takes place, great clouds of ashes, dust, gas, and steam rise from the crater. Loud noises come from the crater. After sometime, white hot molten rock, called lava, begins to flow down the mountain sides. This may go on for days or weeks. Then the volcano will "go to sleep" again for many years. Most volcanoes are found near the seas.
On the basis of your reading of the above passage, fill in the blank with an appropriate word: The mouth of the opening is called the _____ of the volcano.