You can know that Source A is supporting the British war effort because the text mentions the importance of tanks in the war effort and encourages people to help build tanks by purchasing war bonds and saving metal. The language used in the text, such as "travelling fortress," "clears the way for soldiers," and "saves lives," suggests that the tanks are seen as a valuable and necessary part of the war effort. The mention of war bonds and saving metal also indicates that the source is encouraging people to support the war financially. All of these factors suggest that Source A is supporting the British war effort.