完形填空
Six years ago, I walked into a local animal shelter on a cold, rainy day.1my house was already full of cats and dogs, I still couldn't 2an occasional visit to the shelter. This time as I was walking down the row of pens(畜栏) full of barking dogs, I3a small boy looking into one of them.
He had a malnourished（营养不良的）looking body and a4face with a big nose5out of the middle of it. He was looking at a small golden dog. Its ribs（肋骨）were showing and its thin tail was folded between its legs. It wasn't begging for6like the other puppies7was hiding in the corner instead. The boy looked up at me with8eyes and asked what was wrong with it. “It was probably abused and9 ” I said. “That is why it is so 10 ”
The boy 11turned to an old lady and said, “ Grandma, I want this one.” The old woman smiled and walked to the front desk to fill out the 12forms. I followed her and asked 13the boy. She told me that he'd had a 14start in life, too. She had just gotten custody(监护权) of him15the state had taken him from his parents. She hoped getting a dog would make his life 16for him. I looked back at him, holding his scared puppy, and said a prayer for them both.
It was yesterday that I was driving down the road enjoying a gloriously sunny, spring day. Then in a green front yard I saw the boy. His 17body was tall and strong. His nose no longer 18big but fit well into his smiling face. He was throwing a ball to a healthy, happy dog shining golden hair. His little act of love and 19had indeed gone a long way. It had saved a dog's life, 20his own heart, and given this middle-aged man fresh hope for the whole world.
分子上各有3个酶切位点，如图中箭头所指，如果线性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；环状DNA分
子在3个酶切位点上都被该酶切断，则会产生e、f、g三种不同长度的DNA片段。现有上述多个线性DNA分子和环状DNA分子，若在每个DNA分子上至少有1个酶切位点被该酶切断，则从理论上讲，经该酶切后，这些线性DNA分子和环状DNA分子最多能产生长度不同的DNA片段种类数各是( )