TOEFL Reading: ETS-TOEFL阅读机经 - N2LQ2E6L_7X83SIBL$

According to paragraphs 5 and 6, temperate lakes are in general more productive than tropical lakes because A. temperate lakes receive more nutrients from incoming streams B. the summer thermocline is relatively deep in temperate lakes C. solar radiation penetrates more of the water in temperate lakes D. temperate lakes experience vertical circulation of water