grammar--connectives

in my opinion, C seems like a better answer than D...
i also chose C...cuz i think the first sentence needs an explanation or elaboration or what, to state the reason why it is unreasonable to do so. if we put in furthermore in the blank, it seems their relation is just a progression...but if we put in after all, its to support the first sentence, which to me sounds more logical and intact...
即係好似你變番中文, 你要求佢9點前番到屋企係唔合理, 再者佢已經18歲, 同 你要求佢9點前番到屋企唔合理, 到底佢都已經18歲, 好明顯第二個岩啲......

there is not a fixed answer to this kind of questions...im just gving my opinion for your reference, hope its gonna help:)

Thank you. I think the book shows a wrong answer.

yingyingkingdom has given a good explanation. C is the answer.