Farmers know that it is necessary to make hay while the sun shines.

VOA: special.2010.01.31

Why? Because good will and idealism, while necessary,they are not sufficient.

为什么呢？因为美好的意愿和理想主义，虽然必要，但却是不够的。

哈佛公开课 - 幸福课课程节选

While not stipulating dollar amounts of U.S.military aid, it authorizes such sums that are necessary to help the country deal with security needs.

VOA: standard.2009.09.24

Grayson says that while Felipe Calderon's campaign against the powerful drug lords is a necessary effort to protect the nation from criminal enterprises, it will not end narcotics trafficking.

VOA: standard.2009.06.09