代数式ax2＋bx＋c，当x＝1 时值为0 ，当x＝2 时值为3 ，当x＝－3 时值为28 ，则这个代数式是_________ 。
2x2－3x＋1
______ is _____ the students to find out the meaning and the usage of new words.
A．What a dictionary does; help
B．What dictionary does; to help
C．How a dictionary does; help
D．All that a dictionary does; helps
1947年6月，美国抛出马歇尔计划，成为其发动“冷战”的序曲。( )
