请在第(50)处填上正确答案。

A．There have been many attempts to redesign the periodic table since Dmitri Mendeleev drew it up in 1871.

B．Railsback has still ordered the elements according to the number of protons they have.

C．"1 imagine this would be good for undergraduates. "

D．Railsback has listed some elements more than once.

E．And the size of element's symbol reflects how much of it is found in the Earth's crust.

F．The traditional periodic table was well drawn.

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【解析】空格前讲到Railsback还包含了一些symbol(符号)，空格处(本文的最后一句话)必须与前一句有所呼应，因为此处已经没有下文了，故E选项(元素符号的大小也说明了在地壳中找到的数量大小)是正确答案。

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请在第__(51)__处填上正确答案。

A．know

B．have known

C．need know

D．need to know

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A．with

B．in

C．on

D．at

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A．diet

B．meals

C．food

D．dinner

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请在第__(50)__处填上正确答案。

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