植物的根受到地球引力作用，总是向着同一方向生长，这种特性称为____。

 Tim was reading a book at office. Someone knocked on the door. He opened the door.____  .
"My name is Rebecca," said the lady. "You've got to help me!"
"Take it easy." ____ "What happened?"
"My grandma died last month. She left all her money, about $1,000,000 to me. She told me she had saved the money in a bank.____ . "
"But yesterday when I went to the bank, I only got an envelope with nothing in it. he money is gone!"
Rebecca took the envelope out of her handbag and gave it to Tim.
Tim looked at it carefully under the light. It was just an envelope with two old stamps on it. ____ 
“Don't worry, Rebecca," he said. "Your money is still here.”

如图，抛物线y=﹣ $\frac{2}{3}$ x²+bx+c经过A（﹣1，0），B（0，2）两点，将△OAB绕点B逆时针旋转90°后得到△O′A′B′，点A落到点A′的位置．

 下列实验操作错误的是（　　）