I have the following data-set on python

```
import pandas as pd
bcw = pd.read_csv('http://archive.ics.uci.edu/ml/machine-learning-databases/breast-cancer-wisconsin/breast-cancer-wisconsin.data', header=None)

```

Lines like 24 have missing values:

```
1057013,8,4,5,1,2,?,7,3,1,4

```

On `column 7`, there is a `'?'`, and I want to drop this line. How can I achieve this?