The correct options are
B Agricultural practices had fully developed before the Megalithic age.
C The practice of hunting and gathering stopped as agriculture became the predominant practice.
D Some of the important crops cultivated were coffee, oilseeds, and opium.
Before the Megalithic period, agriculture was not a mature practice and experiments were still being conducted. However, in the Megalithic period, agriculture became widespread with the help of irrigation. Hence, megalithic sites were found near river banks or deltas where water for irrigation was abundantly available. Some of the important crops that were cultivated were rice and millets. Despite the practice of agriculture ensuring sufficient food, hunting and gathering was still practiced.