Read the following passage and mark the letter A, B, C, or D to indicate the best option for each of the blanks.

From the seeds themselves to the machinery, fertilizers and pesticides - The Green Revolution regimen depend heavily on technology. One (17) ________, however, depends much more on technology - organic farming. Many organic farmers use machinery, but (18) _______ chemical fertilizers or pesticides. (19) ________ chemical soil enrichers, they use animal manure and plant parts not used as food - natural, organic fertilizers that are clearly a renewable (20) _______. Organic farmers also use alternatives for pesticides; for example, they may rely (21) _______ natural predators of certain insect pests. Then, the need arises, they can buy the eggs and larvae of these natural predators and introduce them into their crop fields.