题干

已知某种限制性内切酶在一线性DNA分子上有3个酶切位点,如图中箭头所指,如果该线性DNA分子在3个酶切位点上都被该酶切断,则会产生a、b、c、d四种不同长度的DNA片段。现有多个上述线性DNA分子,若在每个DNA分子上至少有一个酶切位点被该酶切断,则理论上讲,经该酶切后,这些线性DNA分子最多能产生长度不同的DNA片段种类数是                    (  )

A:3

B:4

C:9

D:12

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C

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    It was a hot,humid day, and my brother Walt and I had decided that the only way to surviveit would be to go swimming in a deep swimming hole across Mr. Blickez's pasture(牧场) and through some woods.

    The onlyproblem with our plan was that this pasture was guarded by a huge, meanHereford bull. Mr. Blickez had told us that Elsie was the meanestbull in the township, maybe even the county, and we believed him. But the hotter it got, the more we thoughtthere was something fishy about his claim. For one thing, we remembered Mr. Blickez liked telling tall tales; for another, Elsie seemed like an oddname for a bull.

    Finally, Italked Mom into asking permission for us to walk through the pasture, but thenanother problem surfaced.Mom said she would talk to Mr. Blickez if we would take our cousin Joanie along with us. Joanie was almost two years older than me anda head taller. If her teasingever got around my grade school, it would be all over for me. In fact, I still had a headache from a quarrelwith her that morning.“I'm not goingswimming with that dumb girl cousin.” I told my mom.

  “Either Joaniegoes with, or you stay home alone,” Mom said in her serious tone. I gave in and we set out. On our way across the pasture, Walt yelledsuddenly. Elsie hadapproached him quietly and was licking(舔) his back. Joanie and Idove under the wire fence, but while I was on the ground I looked up and sawthat Elsie wasn't a big mean bull after all. She was going to keep licking my brother's back as long as he stoodstill.

    We had manygood days growing up and visiting our secret swimming hole guarded by theso-called “big mean bull”. And as it turned out, for a girl cousin, Joanie hasn't been too bad. She's been one of my best friends over theyears.