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【单选题】
X线管内电子轰击靶面的面积称A.
主焦点
B.
副焦点
C.
有效焦点
D.
实际焦点
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举一反三
【单选题】听力原文：W: The floor is awfully wet. What happened? M: No sooner had I gotten into the shower than the phone rang. Q: What does the man imply? (15)
A.
He got out of the shower to answer the phone.
B.
He didn't hear the phone ringing.
C.
There's something wrong with the shower.
D.
He took a shower earlier than usual.
【单选题】The Ground is wet all over, so it must have rained last night, ________? [ ]
A.
mustn't it
B.
wasn't it
C.
hasn't it
D.
didn't it
【单选题】-The ground is wet. -It must have rained last night,_____? [ ]
A.
hasn't it
B.
didn't it
C.
mustn't it
D.
isn't it
【单选题】对yꞌꞌ(t) -6yꞌ(t)-9y(t) =3fꞌ(t)-4 f(t),若令wꞌꞌ(t) -6wꞌ(t)-9w(t)=f(t),则可得
A.
f(t)=3wꞌ(t)-4 w(t)
B.
y(t)=wꞌꞌ(t) -6wꞌ(t)-9w(t)
C.
y(t)=3wꞌ(t)-4 w(t)
D.
y(t)=4wꞌ(t)-3 w(t)
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