When referring to the upcoming arts convention, artists complained that their commission rate is too low to generate any profit on sales.

A When referring to the upcoming arts convention, artists complained that
B Referring to the upcoming arts convention, artists complain that
C In reference to the upcoming arts convention, artists complained about
D When referring to the upcoming arts convention, artists complain about
E With reference to the upcoming arts convention, artists complained that

OA is B
What is the OA saying? 'Referring' could begin a sentence?