浸没在水中的种子，能吸水胀大，而不能萌发，其主要原因是（  ）

化简下列各式( 5 6 a 1 3 b − 2 ) ⋅ ( − 3 a − 1 2 b − 1 ) ÷ ( 4 a 2 3 b − 3 ) 1 2 ⋅ a b

阅读理解。
A
    How do you go to school?  
   Sally： My school is not far from my home. It is about three kilometers. I usually walk to school, but sometimes I go to school by bike.
   Nick： My mother usually drives me to school. But when Aunt Lucy is here, I walk with her. It takes me 7 minutes by car or 25 minutes on foot.
   Mary：I go to school on the school bus. The bus comes at 8: 35 am and I get to school at 8: 50 am. I love to take the school bus because my best friend Linda takes it with me. We have lots of fun.

在观察人的口腔上皮细胞的实验中，以下操作错误的是（    ）

The underlined part in the word “grades” is pronounced as______.