28颗星星，每9颗可以穿成一串，可以穿成几串？还剩几颗？

  ____

     Last Monday, a strong man growing a beard(胡须) arrived at Mr. Li's gold shop by taxi. He told the driver to1. He went into the shop and said, “Hand over your money!”

     Mr. Li was busy putting some goods(货物) on the shelf and he didn't hear him clearly. He turned and asked, “2can I do for you, Sir?”

   “I told you to give me your money. I have got a gun in my handbag!”

 Mr. Li gave him all his money. While the strong man was putting the money into his handbag, Mr. Li took out his own gun from behind. Seeing this, the strong man ran out of the shop with Mr. Li 3 him.

    The strong man jumped into the waiting taxi as Mr. Li shouted, “It's a robbery(抢劫)! Stop!" The taxi driver thoug ht Mr. Li wanted to 4 him. So he jumped out of his taxi with his hands5_. Then the strong man 6 with his gun in his hand and pointed it at Mr. Li.  Mr. Li pointed his gun at the strong man, too. For almost a minute, they stood face to face and gun to gun, but 7of them fired, because they had toy guns.

    Suddenly the strong man 8 his gun and ran down the street away. He stopped a passing car and asked for a ride. The two men in the car were9 to take him along. They were policemen. They gave him a ride straight to 10.

    From an early age, Parker always said that he would be an astronaut. But no matter ____much he studied, worked, and took tests, he never passed. Finally, he ____ (grow) over the maximum(最高的) age for taking the test. He would never be able to realize his dream.

    People ____ ( feel )sorry for his dream. Although he____ (tell) about having to give up his dream, Parker continued training and studying, as though he was going to sit the tests next month.

    He carried this on as he got ____ (old), and when he was a very old man he heard that they were doing some very important medical experiments. For those experiments they needed ____ very old astronaut. Parker, who now walked with a stick, was the only old man in the whole world who was trained to fly in a rocket.

    So Parker found ____(he) flying in space at last. The experiments were very ____ (succeed). The knowledge from the experiments helped doctors to cure one of the ____(bad)sicknesses for old people, and Parker was celebrated as a hero. Photos of the astronaut with a walking stick and no____ (tooth) spread all over the world. It showed  that persistent effort always brings reward, even though that reward may not be in the form we had first expected.

一种铁原子的原子核内有26个质子和30个中子，该原子的核外电子数为（   ）