TOEFL Poweprep文法問題

こんにちは。
Powerprepでわからない問題があったので質問させて下さい。

4. Perhaps the most significant postwar trend was the decentralization of cities throughout the United States,
_____ when massive highway-building programs permitted greater suburban growth.

a)and accelerated a phenomenon
b)a phenomenon that accelerated
c)accelerating a phenomenon which,
d)the acceleration of which phenomenon


解答は(b)のa phenomenon that acceleratedです。
私が選んだのは(a)で単純に２文を繋ぐのに接続詞が必要だと思ったからなんです。Perhaps the most significant postwar trend was the decentralization of cities throughout the United States, [and (the trend) accelerated a phenomenon] when massive highway-building programs permitted greater suburban growth.

解答を見た後は、なるほど！a phenomenon はthe decentralization of citiesを修飾してるんだな？と解釈したのですが
(a)が間違っているのがわかりません。
どなたかご教授お願いします。