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

[Ozzy For President]

Dear Mr Micawber or other coaches / members,

"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'."

I first thought about "as" to be the option, but I have the feeling that "it" is missing.

Actually, I thought the correct answer would be "I assure you that you will have no problem with the task because it's as easy as it can be."

Without the second "it", it feels a subject is missing.

Thank you in advance,

Ozzy
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Wealth is a tool of freedom. But the pursuit of wealth is the way to slavery.
Frank Herbert "God Emperor of Dune"