必需: essential; indispensable; vitally need 短语和例子发展工业所必需的原料 raw materials essential for industrial development; 用在最必需的地方 be used where it is most needed; 这些参考书是我们所必需的。 these reference books are
Cd was the unnecessary element of plants . the high concentration of cd in environment was harmful for development and growth of plants Cd是植物的非必需元素,环境中cd含量过高就会影响植物的生长发育。