In the following sentences, there are some blanks which are numbered. Below the sentences you are given a choice of four words (A), (B), (C) and (D), one of which is the most appropriate. Choose the best word as your answer.

It is one of the tragic ironies of our age that the rocket, which could have been the symbol of humanity’s aspirations for the stars, has become one of the weapons threatening to destroy civilization. This state of affairs has —(51)— a difficult moral problem to —(52)—wishing to take an active —(53)— in the development of astronautics, —(54)— almost all research on rockets —(55)— now carried out by military —(56)— and is covered by various — (57)— classifications. The technical problems involved —(58)—designing long range guided missiles are —(59)— identical with those involved in the —(60)— of reconnaissance rockets.