We have reasonable excuse to refuse.

我们完全有理由拒绝。

She cannot but find a reasonable excuse for her being late.

她不得不给自己的迟到找一个合理的借口。

Can you give me a reasonable excuse for your being so late?

你能就你的迟到给我一个合理的解释吗？

He can not find a reasonable excuse as to these phenomenon.

对于这些现象他没能找到合理的解释。

He or she did not write any reasonable excuse for taunt. Self-love is totally not a fault.

他也说不出个骂芙蓉姐姐的合理的理由来，自恋绝对不是错。