他解释了拥有Wi-Fi连接的有用性。

He explained the ___usefulness of having a Wi-Fi connection.

他们昨天回来了，我采访了他们，想了解他们对这个程序有用性的看法。

They came back yesterday and I have interviewed them to find out what they think about the ___usefulness of the program.

这项研究旨在衡量不同学习方法的有用性。

The study aims to measure the ___usefulness of different learning methods.