Question: I have a pandas DataFrame `df` that looks like this:

```
A      B
0  ['a','b']
1  ['c','d']
2  ['a','c']
3  ['b','d']
4  ['a','d']

```

Now, I wish to subset `df` by only selecting those rows in which `'a'` belongs to the list in `B`, the desired output being:

```
A      B
0  ['a','b']
2  ['a','c'] 
4  ['a','d']

```

Naively, I tried `df['a' in df['B']]`, but this doesn't seem to work. How would I go about doing this?

Answer: 
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