Which key sentence from the passage identifies this
text as a primary source?

Magma forms by partial melting of upper mantle and crust. partial melt means that only a fraction of the available material forms a melt, and that the remainder stays solid. the partial melt rises because of its lower density and ascends through he crust. this is so conclusive mean the answer would be magma igneous rocks or balistic magma.

None of them. they are likely to occur on any plate boundary

Well, a and b together would be negative. b alone would also be negative.

You can google it. the answer is d. international recognition