why as to be used in this particular sentence?

[Wowuik]

Business English Lesson, Advanced Level

ESL/EFL Test #86 "Verbal Communication Skills", question 10

I assure you that you will have no problem with the task because it's as easy ......... can be.

(a) as
(b) than
(c) and
(d) it

Business English Lesson, Advanced Level

ESL/EFL Test #86 "Verbal Communication Skills", answer 10

I assure you that you will have no problem with the task because it's as easy as can be.

Correct answer: (a) as

Your answer was: incorrect
I assure you that you will have no problem with the task because it's as easy it can be.
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why as to be used in this particular sentence?

Wowuik

[Mister Micawber]

.
It is part of the comparative couplet, 'as .... as'-- 'as happy as a lark', 'as long as you want to', 'as easy as can be'.
.
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Native English teacher at Mister Micawber's