I am an apple. I'm a kind of sweet1You can find me grow in many places of the world, but I can't grow well in very hot places2 very cold places.

    I am not like my cousins 3and bananas. I am rounder than a pear and bananas are4than me. I am delicious,5people all over the world like to eat me. They can usually eat me6washing me. They can also cook me to 7all kinds of food, sometimes they put me in cans（罐头）to send me to other places of the world. Almost everyone can have 8 to buy me because I'm so cheap.

    I can also help people get9 There is a saying10English: An apple a day keeps the doctor away. Don't you think that I'm very great? I have many vitamins（维生素）inside, they are 11for you. I really wish to make you happy and healthy. so don't forget to 12me as often as possible（尽可能经常地）.

    But I also need your13, if you want to make me taste better, you must14 me well. Thank you for your love. I'll try to grow well and15sweeter. Then you will like to eat me a lot more.

已知正方体$ABCD-A_1{B}_1{C}_1{D}_1$的棱长为$a$，$\overrightarrow{AM}=\frac{1}{2}\overrightarrow{MC}$，点N为$B{B}_1$的中点，则$\left|\overrightarrow{MN}\right|$=（  ）