In each of the following questions, two sentences I and II are given. Each sentence has a blank in it. Five words (1), (2), (3), (4) and (5) are suggested. Out of these, only one fits at both the places in the context of each sentence. Number of that word is the answer.

I. A troubled student and his PhD guide ............ away for websites to reduce the incidence of spam which troubled internet users in the 2000s.

II. One of the biggest drawback of concrete is that it often develop cracks and scientists have now ......... self-healing concrete.