C#，如何将间距添加到SpeechSynthesizer.SpeakSsml(String)

SpeechSynthesizer synthesizer = new SpeechSynthesizer();

string ssmlText = "<speak version='1.0' xmlns='http://www.w3.org/2001/10/synthesis' xml:lang='en-US'>" +
                  "<prosody rate='medium' volume='default'>" +
                  "<s>This is the first sentence.</s>" +
                  "<break time='500ms'/>" + // 添加间距，500毫秒
                  "<s>This is the second sentence.</s>" +
                  "</prosody>" +
                  "</speak>";

synthesizer.SpeakSsml(ssmlText);