阅读《浣溪沙》，根据要求回答问题。
                                                                                                                                 浣溪沙
                                                                                                                                   晏殊
                                                                                           一曲新词酒一杯。去年天气旧亭台。夕阳西下几时回？
                                                                                           无可奈何花落去，似曾相识燕归来。小园香径独徘徊。

若4x²+2kx+9是完全平方式，则常数k=____． 

维持生态平衡必不可少的生物是（　　）