A、She had already finished them. B、His mother had taken them away. C、She forgot where she had left them. D、Someone in his family

All her life, my mother wanted busy children. It was very important that her house should remain at all times clean and tidy.
    You could turn your back for a moment in my mother’s house, leave a half-written letter on the dining room table, a magazine open on the chair, and turn around to find my mother had "put it back where it belonged", as she explained.
    My wife, on one of her first visits to my mother’s house, placed a packet of biscuits on an end table and went to the kitchen to fetch a drink. When she returned, she found the packet had been removed. Confused, she set down her drink and went back to the kitchen for more biscuits only to return to find that her drink had disappeared. Up to then she had guessed that everyone in my family held onto their drinks, so as not to make water rings on the end table. Now she knows better.
    These disappearances had a confusing effect on our family. We were all inclined to forgetfulness, and it was common for one of us, upon returning from the bathroom, to find that every sign of his work in progress had disappeared suddenly. "Do you remember what I was doing?" was a question frequently asked, but rarely answered.
    Now my sister has developed a second-hand love of clean windows, and my brother does the cleaning in his house, perhaps to avoid having to be the one to lift his feet. I try not to think about it too much, but I have at this later time started to dust the furniture once a week.
Questions 16 to 18 are based on the passage you have just heard.
16. Which is true about the speaker’s mother?
17. Why couldn’t the speaker’s wife find her biscuits and drink in his mother’s house?
18. What does the passage mainly tell us?