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问答题
生物膜法对水质水量变化有较强的适应性
答案：
是的，生物膜法对水质水量变化具有较强的适应性。生物膜法是一种利用微生物附着在固定或流动的介质表面上形成的生物膜来处理废水...
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单项选择题
from scrapy.selector import Selector htmlText=" A1 B1 <c> C1 </c> <d> D <e> E </e> </d> B2 <c> C2 </c>" selector=Selector(text=htmlText) s=selector.xpath("//a/preceding-sibling::*") print(s.extract()) s=selector.xpath("//b/preceding-sibling::*[position()=1]") print(s.extract()) s=selector.xpath("//b[position()=2]/preceding-sibling::*") print(s.extract()) ___________________________________________________________________ print(s.extract()) 程序结果： [] [" A1", " <d> D <e> E </e> </d>"] [" A1", " B1", " <c> C1 </c>", " <d> D <e> E </e> </d>"] [" <d> D <e> E </e> </d>"]
from scrapy.selector import Selector htmlText=" A1 B1 <c> C1 </c> <d> D <e> E </e> </d> B2 <c> C2 </c>" selector=Selector(text=htmlText) s=selector.xpath("//a/preceding-sibling::*") print(s.extract()) s=selector.xpath("//b/preceding-sibling::*[position()=1]") print(s.extract()) s=selector.xpath("//b[position()=2]/preceding-sibling::*") print(s.extract()) ___________________________________________________________________ print(s.extract()) 程序结果： [] [" A1", " <d> D <e> E </e> </d>"] [" A1", " B1", " <c> C1 </c>", " <d> D <e> E </e> </d>"] [" <d> D <e> E </e> </d>"]
A、s=selector.xpath("//b[position()=1]/preceding-sibling::*[position()=2]")
B、s=selector.xpath("//b[position=1]/preceding-sibling::*[position=2]")
C、s=selector.xpath("//b[position=2]/preceding-sibling::*[position=1]")
D、s=selector.xpath("//b[position()=2]/preceding-sibling::*[position()=1]")
A.selector
B.xpath("//a/preceding-sibling::*")
C.extract())
D.xpath("//b/preceding-sibling::*[position()=1]")
E.extract())
F.xpath("//b[position()=2]/preceding-sibling::*")
G.extract())
H.extract())
I.xpath("//b[position()=1]/preceding-sibling::*[position()=2]")
J.xpath("//b[position=1]/preceding-sibling::*[position=2]")
K.xpath("//b[position=2]/preceding-sibling::*[position=1]")
L.xpath("//b[position()=2]/preceding-sibling::*[position()=1]")
B.xpath("//a/preceding-sibling::*")
C.extract())
D.xpath("//b/preceding-sibling::*[position()=1]")
E.extract())
F.xpath("//b[position()=2]/preceding-sibling::*")
G.extract())
H.extract())
I.xpath("//b[position()=1]/preceding-sibling::*[position()=2]")
J.xpath("//b[position=1]/preceding-sibling::*[position=2]")
K.xpath("//b[position=2]/preceding-sibling::*[position=1]")
L.xpath("//b[position()=2]/preceding-sibling::*[position()=1]")
