执行完以下代码后，df_filled 中索引为 2（第三行，原数据为 'forty', 5500）的那一行数据是什么？ import pandas as pd import numpy as np df = pd.DataFrame({ 'age': ['25', '30', 'forty', '28', '-5', '150'], 'income': [5000, 6000, 5500, -2000, 7000, 8000] }) # 1. 清洗 age 列 def clean_age(val): try: v = int(val) if 0 <= v <= 120: return v else: return np.nan except: return np.nan df['age'] = df['age'].apply(clean_age) # 2. 清洗 income 列 df.loc[df['income'] < 0, 'income'] = np.nan df['income'] = df['income'].astype(float) # 3. 填充 df_filled = df.fillna(method='ffill') # 前向填充