He must be ill,____________he looks so pale.A.since B.becaus

as,because,for,since都可以用来表示“因为”.它们之间的区别主要体现在表示原因的程度上,依次为 because-since-as- for
以下回答内容大部分摘自新浪博客
because
because引导原因状语从句语气最强,表示直接原因,常用来回答由why提出的问题,所引导从句可放在主句前,也可放在主句后.例如：
"Why did you do it?" "Because Carlos told me to".
We can't go to Julia's party because we're going away that weekend.
Because he didn't catch the last bus,he had to walk home.
Why are you late for school Because I missed the first bus.
as
as “由于,鉴于” 主从并重,从句说明原因,主语说明结果.说明比较明显的原因.多用于日常语中,常译为“由于”,从句一般放于主句前.例如：
You can go first as you're the oldest.
As you don't feel well,you had better stay at home.
As he wasn't ready in time,we went without him.
I must stop writing now,as I have rather a lot of work to do.
As it was getting late,I decided to book into a hotel.As it is raining,you'd better take a taxi.
since
since “因为,既然” 侧重主句,主句表示显然的或已知的理由.强调说话者和听者双方都知道的原因,常译为“既然”,一般用在这样的句子里.例如：
Since we've got a few minutes to wait for the train,let's have a cup of coffee.
Since you are not feeling well,you may stay at home.
Since everybody is here,let's begin the meeting.
Since I am a boy,let me carry the case.
Since/As you are busy,you had better not go with us.
Since we have no money,we can't buy that vase.
for
for “因为,由于”表明附加或推断的理由,引导的分句前常有逗号.for分句不放句首.是表示附加或推断的理由.例如：
Day breaks,for the cock crows.
It must have rained last night,for the ground is wet.
I won't go in,for my father is there.
He must be ill,for he is absent.
I went to see him,for I had something to tell him.
题目中正是根据他脸色苍白推断的他身体不好啊

答案可以是because，as和for。。。只有since是错的