Why did president truman feel he had no choice but to drop the atomic bombs?

President Truman felt he had no choice but to drop the atomic bombs because he felt that without the bomb, a costly invasion would be necessary.

Since Truman's advisers warned him that any attempt to invade Japan would lead to appaling American casualties, President Harry S. Truman ordered that the new weapon be implemented to quickly end the war. On August 6, 1945, the American bomber Enola Gay dropped a five-ton bomb over the Japanese city of Hiroshima.13 nov. 2009

the three answers (for plato at least) are:

b-condemned sola fida 

d-set rules for establishing seminaries and for educating clerics in the seminaries 

f-recommended moderation in the sale of indulgences and banned the sale of false indulgences 

b. sec, foreign relations