Designing a resource allocation task where allocating more resource to one person results in less to

Hi there, Im a little stuck with regard to designing a question in the survey platform.

This is the question/task that I'd like my survey participants to answer:

"This task requires you to allocate a total of 15 * sessions to two different clients. Each of the columns in the table below add up to 15. Select a letter from the table below which indicates the column you have chosen to allocate the 15 * sessions".
a b c d e f g h i j k l m n
Client A 1 2 3 4 5 6 7 8 9 10 11 12 13 14
Client B 14 13 12 11 10 9 8 7 6 5 4 3 2 1

I've tied the slider and multiple choice question type options but cant get them to work. As a newbie to this Im hoping there might be a simple answer...Cheers