Final answer:
The mainland of Southeast Asia extends from the Chinese border to the tip of Myanmar. Myanmar is the largest country on the Southeast Asian mainland and it has a central mass with a southern protrusion that borders Thailand toward the Malay Peninsula. The northern border area between India and China has high mountains that are part of the Himalayas.
Step-by-step explanation:
The mainland of Southeast Asia extends from the Chinese border to the tip of Myanmar. Myanmar is the largest country on the Southeast Asian mainland and is about the same size as the US state of Texas. The country has a central mass with a southern protrusion that borders Thailand toward the Malay Peninsula. The northern border area between India and China has high mountains that are part of the Himalayas. The Irrawaddy River cuts through the center of Myanmar from north to south.