Which sentence is punctuated correctly? i crouched low covered my mouth and nose and braced for impact. after the avalanche stopped, we dug ourselves out of the barricade. that morning began without a hitch, as we headed up the mountain. without warning, a massive wall of snow, suddenly slid above us.

The answer is "After the avalanche stopped, we dug ourselves out of the barricade."

not sure how to answer your question sorry

quite an interesting way to teach topic sentences, because the answer to this question is the best definition of a topic sentence that one can present, it is as if answering the questions taught you how to do it, this is some real teaching, it is terribly well-done.

the topic sentence in this paragraph is:

topic sentences writers stay on track and keep a paragraph from becoming an aimless collection of sentences about an idea.

which incidentally tells you what a topic sentence should be.