Comment on the fact that at present in south africa it is legal for big game hunters to shoot rhinos as trophies.

It is true that in South Africa (and also Namibia) it is still legal for big game hunters to shoot rhinos for trophies but this allowance is strictly limited to 5 rhinos per year for both countries and these are only for old, post-reproductive bulls. This small quota is most likely related to the fact that 3 out of 5 rhino species is on the critically endangered species list. 

They both determine the exact age of rocks.