选词填空。（注意大小写）

yesterday    today    tomorrow  

1.I went to school by bus ________.

2.________ he writes a letter to his sister.

3.She will go to the park ________.

4.My parents bought many lights ________.

5.I'm going to visit my aunt ________.

起重机的使用频繁程度，称为起重机的（）。