Subsistence farming is the dominant economic activity in which of the following countries?

Subsistence farming is not used anymore, only occasionally in certain parts of the world but in the 1800's it was used in the U.S. by people who only planted enough to get them by, and if they had the smallest portion left they traded it, typically for alcohol 

Developing countries such as parts of south America, Asia, and Africa

81 is a composite number

Magma is outside the volcano  and lava is inside the volcano.