Placer mining involves to extract minerals.
a. removing the top layer of earth
b. tunneling under the earth
c. dredging sediments from the water bottom
d. none of the above

Placer mining involves dredging sediments from the water bottom to extract minerals.  Correct C

This ancient method uses water to excavate, transport, concentrate, and recover heavy minerals from alluvial or placer deposits.

Typical minerals recovered in placers are minerals of high specific gravity and physical toughness, like gold, platinum, diamonds...

since there isn't any suggested answers i will give you the reason and hopefully you can figure it

south africa, or rather all of africa, has not been in the location in which it is nowadays throughout the geological past. after the super-continent pangaea started to break apart, gondwanaland, in which was africa, moved further south, and parts of it came to be very close to the south pole. because the climate in this part of the world is very cold due to the very dispersed sun rays in there, what is now south africa, started to freeze, and the conditions were right for the formation of glaciers. the glaciers existed up until the period when gondwanaland started to break apart and what is now africa started to move towards north, sp it warmed up and the glaciers melted.