Casually vs. mildly

[Jincdong]

Business English Lesson, Advanced Level

ESL/EFL Test #174 "Organizing a Business", question 1

According to Ron Kurtus of School for Champions, some companies may be ......... successful, but they could be even more profitable if they paid attention to the basics of organization.

(a) casually
(b) mildly
(c) scarcely
(d) willfully

Business English Lesson, Advanced Level

ESL/EFL Test #174 "Organizing a Business", answer 1

According to Ron Kurtus of School for Champions, some companies may be mildly successful, but they could be even more profitable if they paid attention to the basics of organization.

Correct answer: (b) mildly

Your answer was: incorrect
According to Ron Kurtus of School for Champions, some companies may be casually successful, but they could be even more profitable if they paid attention to the basics of organization.
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I cann't completely understand this sentence.
Can you help me why the correct answer is mildly?

[Mister Micawber]

.
Mildly here means 'modestly, moderately'. The key to finding the correct answer is in the phraseology, 'but...even more profitable [=successful]...'.

'Casually successful' is not an image that I can imagine; the words do not collocate highly.
.
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Native English teacher at Mister Micawber's