Meaning of 'what on earth?'

[Sajjad Qamar]

Test No. incompl/elem-28 "Responses", question 5

Mike: 'More people die from eating fatty food than from smoking.
Jane: '.........'

(a) You must be ridiculous!
(b) You must be laughing!
(c) You must be smiling!
(d) You must be joking!

Test No. incompl/elem-28 "Responses", answer 5

Mike: 'More people die from eating fatty food than from smoking.
Jane: 'You must be joking!'

Correct answer: (d) You must be joking!

Your answer was: correct
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What does What,s on earth for means?

[Torsten]

You probably are referring to this question:
http://www.english-test.net/esl/learn/english/grammar/ei028/esl-test.php#four

What on earth... is a phrase that conveys surprise.
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Test Of English for International Communication
TOEIC Preparation & TOEIC Vocabulary

[Watie]

Dear Torsten..

how do we use "You must be ridiculous"?

Thanks..