I know that is very impolite and must have caused you much trouble, but I ’ d like to explain myself to you. On my way to your office, an old lady suddenly fell to the ground due to a heart attack. I stopped a taxi and took her to a nearby hospital. I stayed there until her son came, which spoiled our appointment. The passage excerpts from __________. A. a letter of recommendation B. a letter of apology C. a letter of thanks D. a letter of invitation