右图为生物膜简明结构模式图，a、b表示两种物质，有关说法正确的是（   ）

Dear Daming，

    Thanks for your letter and the wonderful photos. I like them very much.

Here are some of my photos. I'm sending them to you . In the first photo，it is snowing. I'm making a snowman with my friends. In the second one，the sun is shining and the wind is blowing. I'm flying a kite in the park. In the third one，I'm swimming in the sea. In the last one，I am with my sister Lucy. We're playing chess.

Best wishes to you and your family.

Love，

Jack

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