4 sentences

1. 氧气是物质燃烧所必需的条件。

   yǎng​qì shì wù​zhì rán​shāo suǒ bì​xū de tiáo​jiàn.

   Oxygen is a necessary condition for combustion.

   Source: Dong Chinese Dictionary

2. 硒是一种人体必需的微量元素。

   xī shì yī zhǒng rén​tǐ bì​xū de wēi​liàng yuán​sù.

   Selenium is an essential trace element for the human body.

   Source: Dong Chinese Dictionary

3. 这项工程必需大量的资金投入。

   zhè xiàng gōng​chéng bì​xū dà​liàng de zī​jīn tóu​rù.

   This project requires a large amount of capital investment.

   Source: Dong Chinese Dictionary

4. 铁、锌和碘是人体必需的微量元素。

   tiě, xīn hé diǎn shì rén​tǐ bì​xū de wēi​liàng​yuán​sù.

   Iron, zinc, and iodine are essential trace elements for the human body.

   Source: Dong Chinese Dictionary