This task is about writing a correct answer for the reading comprehension task. Based on the information provided in a given passage, you should identify the shortest continuous text span from the passage that serves as an answer to the given question. Avoid answers that are incorrect or provides incomplete justification for the question.

[Q]: Passage: In the mid-1950s, Frank Burnet, inspired by a suggestion made by Niels Jerne, formulated the clonal selection theory (CST) of immunity. On the basis of CST, Burnet developed a theory of how an immune response is triggered according to the self/nonself distinction: "self" constituents (constituents of the body) do not trigger destructive immune responses, while "nonself" entities (e.g., pathogens, an allograft) trigger a destructive immune response. The theory was later modified to reflect new discoveries regarding histocompatibility or the complex "two-signal" activation of T cells. The self/nonself theory of immunity and the self/nonself vocabulary have been criticized, but remain very influential. Question: What property of T cells caused modification of CST?
[A]: the complex "two-signal" activation


[Q]: Passage: The tradition holds that Virgil was born in the village of Andes, near Mantua in Cisalpine Gaul. Analysis of his name has led to beliefs that he descended from earlier Roman colonists. Modern speculation ultimately is not supported by narrative evidence either from his own writings or his later biographers. Macrobius says that Virgil's father was of a humble background; however, scholars generally believe that Virgil was from an equestrian landowning family which could afford to give him an education. He attended schools in Cremona, Mediolanum, Rome and Naples. After considering briefly a career in rhetoric and law, the young Virgil turned his talents to poetry. Question: In which cities did Virgil attend schools?
[A]: Cremona, Mediolanum, Rome and Naples


[Q]: Passage: In the spring of 1854 the Russians again advanced, crossing the Danube River into the Turkish province of Dobruja. By April 1854, the Russians had reached the lines of Trajan's Wall where they were finally halted. In the center, the Russian forces crossed the Danube and laid siege to Silistra from 14 April with 60,000 troops, the defenders with 15,000 had supplies for three months.:415 The siege was lifted on 23 June 1854. The English and French forces at this time were unable to take the field for lack of equipment.:415 Question: How man troops did the Russians have when they attacked Silistra ?
[A]:
60,000 troops