Definition: In this task, you're given a question, along with a context passage. The passage will not have a direct answer to the question, but can include some information and a clue for the answer. Assuming you can only obtain information about one entity in the passage, your job is to determine whether information from the passage can be used to answer the question. Indicate your choice as `a` for Yes or `b` for No.
Input: Question: Who was the perpetrator of the event that caused the capital to be in mourning? Passage:On May 10, United States President Andrew Johnson had declared that the rebellion and armed resistance was virtually at an end, and had made plans with government authorities for a formal review to honor the troops. One of his side goals was to change the mood of the capital, which was still in mourning following the assassination of United States President Abraham Lincoln a month before at Ford's Theater. Three of the leading Federal armies were close enough to participate in the procession. The Army of the Tennessee arrived via train. The Army of Georgia, also under the command of William T. Sherman, had just completed its Carolinas Campaign and had accepted the surrender of the largest remaining Confederate army, that of Joseph E. Johnston. It arrived from North Carolina in mid-May and camped around the capital city in various locations, across the Potomac River from the Army of the Potomac, fresh off its victories over Robert E. Lee in Virginia. It had arrived in Washington on May 12. Officers in the three armies who had not seen each other for some time (in some cases since before the war) communed and renewed acquaintances, while at times, the common infantrymen engaged in verbal sparring (and sometimes fisticuffs) in the town's taverns and bars over which army was superior. Sherman, concerned that his Westerners would not present as polished an image as the eastern army, drilled his forces and insisted that uniforms be cleaned, buttons and brass shined, and that bayonets glistened.


b