The population explosion is the result of achievements in modern science which have drastically reduced the death rate.
結果 is result of 原因
原因 results in 結果
関係詞whichについて(授業で触れてない部分だけど、補足説明)
The population explosion is the result of achievements in modern science.
Modern science have drastically reduced the death rate.
下の文章を、modern scienceについての説明として、上の文章に加えたものが今回の文章
同じ名詞に注目して、2つ目の文章ではそれをwhichに変えて繋げる
The population explosion is the result of achievements in modern science
Modern science(which) have drastically reduced the death rate.