intense design and development effort ___to the introduction of electronic exchanges.为什么用has been applied不用have been applied?

intense design and development effort ___to the introduction of electronic exchanges.
为什么用has been applied不用have been applied?

看我的 这个句子的主语是 effort 所以用单数
design development都是修饰 effort

Automation refers to the introduction of electronic control and automatic probably implies that _. A) wages should be paid on the basis of