Why is control difficult when a large amount of black is added directly to white to make grey poster colour?

Correct answer: Black is highly potent, so the mixture becomes dark very quickly

This is an example of tinting strength in colour mixing. Black is much stronger than white in changing value, so a large addition can make the mixture dark suddenly rather than gradually. Option A is correct. The practical method is to begin with white and add tiny amounts of black while mixing and checking the value. White does not make black transparent, and the other claims are false.