I would say B and D also. A and C make no sense and are irrelevant. Keep using deductive reasoning and have a good day!
Answer:
cause at that time almost everything was natural nothing was a mixture of anything but now we don't know weather the thing is natural or not or is it mixed with something else also
for eg in earlier times fruits were grown naturally but now they r being injected to grow faster which is not natural and might cause us problems in future but the ppl in past didn't
