usual vs. usually

[Honly]

English Grammar Error, Intermediate level

ESL/EFL Test #19 "Adverbs and Gerunds", question 6

They have a tendency to miss the train they wanted and so they'll probably arrive late as usually.

(a) tendency
(b) wanted
(c) usually

English Grammar Error, Intermediate level

ESL/EFL Test #19 "Adverbs and Gerunds", answer 6

They have a tendency to miss the train they wanted and so they'll probably arrive late as usual.

Correct entry: usual
The error was: (c) usually
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WHICH OF THESE IS CORRECT ? AND WHY?

Honly

[Mister Micawber]

.
Usually is wrong; the expression is 'as usual'. The others are correct, although wanted could be replace by want.
.
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Native English teacher at Mister Micawber's