Read the text and determine if the sentence is true:

Mickey's Birthday Party is a theatrical cartoon short made by Walt Disney and distributed by RKO Radio Pictures. It was released on February 7, 1942. The animated film was directed by Riley Thomson and animated by Les Clark, James Moore, Ken Muse, Armin Shaffair, Riley Thompson, Bernie Wolf, and Marvin Woodward.

Sentence: Les Clark had been doing animation for Disney for years
It's impossible to say