皮皮学，免费搜题
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【简答题】
区别下列名词： （1）σ键和π键 （2）正常共价键和配位共价键 （3）极性键和非极性键 （4）π键和离域π键 （5）等性杂化和不等性杂化 （6）轨道和反键轨道 （7）永久偶极和瞬间偶极 （8）van der Waals力和氢键
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皮皮学刷刷变学霸
举一反三
【单选题】—— What’s wrong with Tom? —— Bad luck! Tom his bike and hurt his leg.
A.
fell
B.
fell off
C.
fell down
D.
fell behind
【单选题】—What happened to Tom? —He ______ his leg while playing football with his classmates.
A.
had broken
B.
has broken
C.
broke
D.
was breaking
【单选题】What's the matter with Tom?
A.
He threw his bike away.
B.
He hurt (使受伤,伤害) his leg.
C.
He sold his bike.
【单选题】---Tom hurt his left leg in the game this morning. ---Really? It would take at least two weeks ______ he could recover and return to school.
A.
when
B.
that
C.
since
D.
before
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