why use "glad" instead of "certain"?

[Ermak]

English Grammar Tests, Elementary Level

ESL/EFL Test #60 "Chit Chat: In the theatre", question 10

Michael Dobson: I'm very ........., ladies you know who I am. Now can you stop talking so that we can get on with the play?

(a) certain
(b) glad
(c) clear
(d) clever

English Grammar Tests, Elementary Level

ESL/EFL Test #60 "Chit Chat: In the theatre", answer 10

Michael Dobson: I'm very glad, ladies you know who I am. Now can you stop talking so that we can get on with the play?

Correct answer: (b) glad

Your answer was: incorrect
Michael Dobson: I'm very certain, ladies you know who I am. Now can you stop talking so that we can get on with the play?
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why use "glad" instead of "certain"?

Ermak

[Conchita]

The author probably wanted to give the sentence a touch or irony, Ermak. Besides, the use of 'certain', though grammatical in the test sentence, isn't natural when we are already aware of something. It would be like saying 'I'm certain you've made soup for supper' when seeing a bowl of steaming soup on the table.