下列关于果胶酶的叙述正确的是（    ）
①能使果汁变得澄清，但不能提高出汁率　
②凡是活细胞都能产生果胶酶　
③果胶酶能将果胶分解成半乳糖醛酸　
④植物、霉菌、酵母菌和细菌均可产生果胶酶　
⑤果胶酶特指分解果胶的一种酶　
⑥纤维素酶和果胶酶可用于去除植物细胞壁　
⑦组成果胶酶的基本单位是氨基酸或核苷酸．

   Dictionaries say that “spring fever” is a lazy or restless feeling . People usually have the feeling on the first warm day of spring. Everyone has probably felt it at one time or another . people have talked about it for years.
   Recently, scientists have taken a good look at spring fever. What they found is quite surprising.
   In many places we live in , the winter months often seem dark and dreary. When spring comes late March , the world becomes colorful . The number of daylight hours grows with each passing day. According to many scientists, all the sunlight sets off chemicals in the brain (大脑). The chemicals make our feeling change and bring on spring fever.