It's generally believed that teaching is _______it is a science.

这句话的意思是：教育与其说是一种科学还不如是一种艺术.选d
as...as意为"和……一样",表示同级的比较.使用时要注意第一个as为副词,第二个as为连词.其基本结构为：as+ adj./ adv.+as.例如：
[解题过程]
(1)This film is as interesting as that one.这部电影和那部电影一样有趣.
(2)Your pen writes as smoothly as mine.你的钢笔书写起来和我的一样流畅.
其否定式为not as/so +adj./ adv.+as.例如：
This dictionary is not as/so useful as you think.这本字典不如你想象的那样有用.
若有修饰成分,如twice,three times,half,a quarter等,则须置于第一个as之前.例如：
Your bag is twice as expensive as mine.你的袋子比我的贵一倍.
几个关于as...as的常见句型：
(1)as...as possible
Please answer my question as soon as possible.请尽快回答我的问题.
(2)as...as usual/before
She looks as pretty as before.她看起来和以前一样漂亮.
(3)as long as...(引导条件状语从句)
It took us as long as three years to carry out the plan.我们花了长达三年的时间才完成这项计划.
(4)as far as
He walked as far as the railway station yesterday evening.昨天傍晚,他一直散步到火车站.
(5)as well as
She cooks as well as her mother does.她烧菜烧得跟她母亲一样好.
一些带有as...as结构的常见短语归纳：
as busy as a bee像蜜蜂一样忙碌
as easy as ABC像ABC一样容易
as deep as a well像井一样深
as light as a feather像羽毛一样轻
as soft as butter像黄油一样软
as rich as a Jew像犹太人一样富裕
as + 形容词或副词原级 + as
1）在否定句或疑问句中可用so… as.
He cannot run so/as fast as you.
2）当as… as 中间有名词时采用以下格式.
as +形容词+ a +单数名词
as + many/much +名词
This is as good an example as the other is.
I can carry as much paper as you can..
3）用表示倍数的词或其他程度副词做修饰语时,放在as的前面.
This room is twice as big as that one.
Your room is the same size as mine.
4) 倍数+ as + adj.+ as 倍数+ then + of
This bridge is three times as long as that one.
This bridge is three times the length of that one.
Your room is twice as large as mine.
Your room is twice the size of mine