已知函数$y=f\left(x\right)$的导函数存在，则函数$y=f\left(x\right)$在一点的导数值为$0$是函数$y=f\left(x\right)$在这点取极值的（    ）

    As a child, I started learning to play to piano, my favorite musical 1 , but I was forced to give up when I started my middle school 2I could concentrate more on my studies.

    It's one of my biggest 3 to stop practicing the piano when I recall sadly today. During the following years, I kept telling my piano teacher that I would 4. However, I didn't keep my promise because I was 5 with my study.6 I lost touch with my teacher. Some years later, my teacher died. I was very sad because I lost such a good teacher. She was a very warm and gentle person. It hurts me to think she may have been 7 that I never returned. I haven't taken lessons since then but to be honest, I 8 to. Sitting at the piano, I couldn't help recalling many 9—times of my practising at home and playing before my teacher and one time my teacher 10 me after I played entire pieces of music wrong in front of her colleagues. I was so 11 that I could hardly say anything. But her 12 helped ease my shame. These memories,13, good or bad, never caused my 14 for playing the piano again.

    This thought then led me to think that 15 is like music, and that we all try to play different 16 in the instrument of our life. Sometimes the pitch(音高) is 17 when we play it well, but sometimes we are out of tone. However, we all continue to create our own 18 style of music. No matter what style our music is, it is 19 that we sing the songs of joy, quietness and love. Though I may never make it back to piano lessons, it doesn't 20 that I've stopped making music.

下列是有关实验试剂在实验操作中的应用：

（1）用海藻酸钠制成人工种皮，可以保护通过____ 技术获得的胚状体；作为包埋剂的海藻酸钠加热溶解后，冷却至室温再和一定浓度的酵母菌溶液混合，用注射器将混合液缓慢而匀速地注射到一定浓度的____ 中，一段时间后得到了固定化的酵母细胞．

（2）用上述过程获得的固定化细胞和一定浓度的葡萄糖溶液混合一段时间后，用　____ 检测其酒精的生成情况．若经过检测发现没有酒精生成，请分析其可能的原因有哪些？____ （写两点）．用上述固定化细胞发酵获得酒精后，再进行工业纯化，形成浓度为____ 的酒精溶液用于根尖的解离，形成浓度为____ 的酒精溶液用于提纯DNA．

（3）在DNA粗提取实验中配制浓度一般为____ 　的NaCl溶液溶解DNA，析出杂质．在豆腐乳的制作过程中加NaCl的注意事项是什么？____