“I Had To”
Those three words might be true but they will never be enough for a response to a question in your viva.
There’s always a reason and it’s always worth digging into. You always need the “because” or the “why” behind doing things a certain way in your PhD.
“I had to” might be the first words that come to mind with something difficult or even something particularly challenging in your research journey. If you know of situations like this ahead of your viva then it will be helpful to consider what else you could say. If it’s a sensitive topic you’ll probably still need to say something more than “I had to” – so think in advance what you might feel comfortable saying.