Solid connection  

saves the stones. If Ba instead, may lead to their capture.

Seki  

And now makes a seki. Black can play a ko but with the risk of losing everything. Hans

Miai Value analysis

Seki  

Let's take this as the zero diagram.

 

After , A and B are miai for Black, so he lives with 7 points. C-D is still Black's sente.

 

Instead of playing the seki, White can also execute - in sente. Later, Black's block will be sente, so we count this as 4 points (5 for Black and 1 White at D).

So this is a sente with value 3, while making the seki is a gote move with miai value 3.5 and therefore better in most circumstances.

White first  

White can kill the whole group this way

White first  

Or this way via bent four in the corner

White first  

This might be the strongest resistance: ko.

Just the stones  

White can also take 8 stones in sente (the follow up would be larger).

If White takes the sente rather than the ko, we get the following game tree

                     -6.25 (9.75)
                    /       \
               3.5 (3.5)   -16 (0|21))
                  / \       / \
                 7   0    -16 (-37)