A question about MGMAT online SC flashcards

[AZ679]

Here is a SC question from MGMAT online flashcards:

"I sent a card thanking my eight grade English teacher for teaching me about .... "

My question is about the role of 'thanking':
Is it, as an adjective, modifying the card (which I do not think so, since a card cannot thank anybody), or is it, as an adverb (or adverbial phrase), modifying the verb 'sent' (or the whole the first sentence)?

If the latter, then isn't a comma necessary before 'thanking'?

I sent a card, thanking my eight grade .....

Best,

[AZ679]

The flash card that I have mentioned is here:
manhattanprep. c o m/gmat/studentcenter/flashcards/gmat-flash-cards.cfm

(I am not allowed to send a URL, so I have changed the .com to . c o m)

In SC flashcards, #12

[RonPurewal]

hi, please post this question in the correct folder (= "MGMAT non-CAT verbal" folder).
thanks.