...the length of time _ the earth to complete one revolution...
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...