อะไรคือขั้นตอนของผู้ชนะ Carlos Sastre เมื่อประเภททั่วไปคือ Denis Menchov?
What is the stage of winner Carlos Sastre when the general classification was Denis Menchov?
df = pd.DataFrame(columns=['stage', 'general_classification', 'winner'])
df.loc[(df['general_classification'] == 'denis menchov') & (df['winner'] == 'carlos sastre'), 'stage']