A.takes
问题描述：
A.takes
B.when takes
C.it takes
D.that takes
为什么选C不选D?
重新输入一遍问题.
A solar year is the length of time _ the earth to complete one revolution round the sun.
答
it takes sb time to do sth
是固定的句子结构,本题的后半句还原出来应该是：
it takes the earth a solar year to complete one revolution round the sun.
而不是：a solar year takes the earth to complete one revolution round the sun.
如果选D,就变成了第二种情况,这明显不对（不符句型结构）
回过头来看整句,其实是：
A solar year is the length of time _(that) it takes_ the earth to complete one revolution round the sun.
注意这里是省略了从句的连词that,所以你下意识的感觉那里该有that.