The last one is punctuated correctly.
ABSOLUTELY. Discoveries come from mistakes. Think about it. If the inventor of a fridge didn't find out that his/her food spoiled, would we have a fridge? No, probably not. Mistakes cause discoveries, and we learn from mistakes.
Answer:
the correct tense for the sentence would be:
each of the students <u>are</u> responsible for studying.
<u><em>hope i helped ! have a nice day (:</em></u>
