Well, that concludes another very fine experiment. which word in the sentence is an interjection?

The answer is: well.

An interjection is a word or utterance that expresses in an immediate way a reaction or a feeling, like in this case, the word “well”; as such, it is not a part of the sentence properly speaking. There are many interjections used in English nowadays, and though they are very heterogeneous, the category includes greetings (hey, hi), curses, doubt markers (um, er), and even responses (okay).  

you   can   protest   and   boicot   and   have   marches  

the answer is b. the tomb is underground and dark, and its opening is covered by a heavy grate.