Assertion: Out of 40,000 km of railways in Africa continent, South Africa alone account for 18,000 km.
Reasoning: South Africa is very rich in minerals like copper, diamond and Gold.

Both Assertion (A) and reasoning (R) are correct and R is the correct explanation of A.

Africa continent, despite being the second largest, has only 40,000 km of railways with South Africa alone accounting for 18,000 km due to the concentration of gold, diamond and copper mining activities.