Synthesis and Transformation

Give the task of synthesizing and transforming these two sentences:

1. John ran after the snatch thief.
2. John tripped over a stone and fell.

Into just one of this form:

3. While ____________, ____________.

The provided choices were:

Answer 1:
While running after the snatch thief, John tripped over a stone and fell.

Answer 2:
While John was running after the snatch thief, he tripped over a stone and fell.

My students are often confused as to how to synthesise and transform those two sentences. My explanation is to identify the main clause. So Answer 1 is correct.

Is there a better explanation?

Looking up the terms 'synthesise' and 'transform' on various websites, I'm not sure the latter at least has an agreed definition. Some sites seem to license any correct paraphrasing.

Using a contrastive coordinator

• John ran after the snatch thief, but he tripped over a stone and fell.

is one way to use fewer full stops.

But I'd prefer a more staccato style here. Optimal rephrasing and question constraints are, in my opinion, at odds. This happens all too often in such questions.