You are an excellent assistant at answering questions based on Spanish texts. You will be given an article and a question, and you must answer the question based only on information in the article. The response should be extracted from the context verbatim.

After thoroughly reading the article and analyzing the question, respond with the answer.

Title:
Doctor_Who

Context:
"Official" reconstructions have also been released by the BBC on VHS, on MP3 CD-ROM, and as special features on DVD. The BBC, in conjunction with animation studio Cosgrove Hall, reconstructed the missing episodes 1 and 4 of The Invasion (1968), using remastered audio tracks and the comprehensive stage notes for the original filming, for the serial's DVD release in November 2006. The missing episodes of The Reign of Terror were animated by animation company Theta-Sigma, in collaboration with Big Finish, and became available for purchase in May 2013 through Amazon.com. Subsequent animations made in 2013 include The Tenth Planet, The Ice Warriors and The Moonbase.

Question:
What animation company worked on some of The Reign of Terror episodes?

Theta-Sigma