下列关于电磁波的说法中，正确的是（ ）
阅读理解。
M：Hi．Linda．
W：Hello．Uncle Tom．It is sunny today．
M：Yes，it is．Welcome to my farm．
W：Thank you very much．Wow!You have a lot of animals．What are those?
M：They are horses．
W：How many horses do you have?
M：Nineteen．
W：Look at these!Are these tomatoes?
M：Yes, they are．
W：They are big and red．
M：Try some．They're good．
W：Thanks．Yum!
关于密度知识，下列说法中正确的是（ ）
