Explain why some partly molten material rises toward earths surface

Sometimes, some partly molten material rises towards the Earth's surface because either it gets added to the magma in the due course of the upward movement of magma and the magma doesn't get enough to melt it completely, or the material consists of elements with extremely high melting points.

Explanation: The magma carries molten material from the mantle towards the surface of the Earth. In its course of rising up, sometimes some materials get added to it after it is just about to reach the Earth's surface. Such material doesn't melt completely and reaches the surface in a partly molten form. In other cases, the magma consists of elements with melting points above 1200° Celsius to 1300° Celsius. Such materials sometimes reach the Earth's surface in a partly molten form.

A,) cause a & b are equal & both gives 180°