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 .....

Thanks,

P.S.
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]

"thanking" describes "a card"; it works like a normal __ing modifier.

since the card contains text that says "thank you", it's not inaccurate to state that the card is thanking the recipient.

[RonPurewal]

the same is true for other documents, and, indeed, for anything else that contains words.

e.g., OG #54: "the report recommended xxxx"

[AZ679]

the same is true for other documents, and, indeed, for anything else that contains words.

e.g., OG #54: "the report recommended xxxx"

Thanks Ron,

So a comma would be necessary if 'thanking' were supposed to modify the verb and the subject.

[tim]

Exactly! You might also want to review the modifiers section in our strategy guide, which describes in detail the difference between adding or omitting a comma in front of an -ing word.