Which two cities lie across the nile from each other?
a. dar es salaam and addis ababa
c. addis ababa and omdurman
b. nairobi and khartoum
d. khartoum and omdurman

Dar es Salam is locate on the shore of the ocean, not Nile, so we can exclude A).

Addis Ababa is the capital of Ethiopia, but it's not placed on the Nile.

So since we have only options b) and c) left, then " Khartoum" must be one of the cities, and it is!
The other city is Omdurman, so the correct answer is d)

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