Wal-Mart Stores' Chinese operations are under scrutiny. An investigation into pork prices has led to staff arrests and stores closed. Days after the scandal became public, the head of Wal-Mart's business in the country quit for "personal reasons" and he is the latest in a number of senior executives to depart in recent months. Sam Webb looks at the retailer's woes and asks what lies ahead for Wal-Mart in China.