Conclusion is: "Since" the new signs have been erected, the riders have had their pockets picked at nearly double the rate than before. We have to look for the change that the signs brought because of which the rate increased by two times.

(A) Since Central Station’s major renovation, during which the signs were erected, Central Station has become much more attractive to tourists from out of town. Does not give any information on how becoming more attractive to tourists=double the rate. Incorrect.

(B) Rising gas prices and a surging downtown job market have caused the daily number of riders at Central Station nearly to double within the past year. Overcrowding may be a possible reason for the pickpocketers to find easy targets. But then, what is the significance of the signs that were erected? Hold but check other options with some connection between signs and rate increase.

(C) Riders walking past the new signs tend to rummage through their pockets or feel through their clothes to verify the presence of their possessions. So the signs made it easier for the pickpocketers to do their jobs. Hold.

(D) The number of individuals convicted of petty theft or grand theft for picking pockets at Central Station has decreased within the past year. Does not directly relate with our conclusion of signs erected=rate increase. Incorrect.

(E) Most of the pickpockets’ victims were riding the subway during peak travel hours, when Central Station is especially crowded. Possible contender. But again, does not relate "signs" with the conclusion. The conclusion clearly states "since the signs were erected. Hence, something must have changed after the signs were erected that made it easier for the pickpocketers and doubled the rate of incidences. Incorrect.