(A) Their invention has been proved to be a boon to the society since many diseases caused by the microbes could be cured by these antibiotics.
(B) The scientists therefore face a continuous challenge to keep inventing newer drugs to counter this problem.
(C) A major hurdle in the use of antibiotics, however, is that the microbes develop resistance to the antibiotics thus rendering these ineffective.
(D) Antibiotics are chemical substances produced by microbes which are capable of inhibiting the growth of other microbes.