<Li> Janel Parrish portrays Mona Vanderwaal, the original and first "A". She is the person who stalked Alison before her disappearance, and did this as revenge for Alison crowning her "Rosewood's biggest loser". After Alison disappeared she became best friends with Hanna Marin and they transformed themselves into the most popular girls at school, taking Alison's place . In the season two finale she is exposed as "A" and sent to Radley Sanitarium, where she is visited by CeCe Drake (Charlotte), who takes Mona's place in the "A" game . In Season 3, she continues working as "A" and by the end of the season is kicked off the "A" team and becomes a victim of "A" along with the other girls . In Season 4 she helps the girls out with their investigations of the new "A". In season 5 she creates an army so as not to be intimidated by Alison's return . Later in the season, she fakes her own death to ferret out "A" but is kidnapped by "A" and imprisoned in "A" 's dollhouse . After the time jump, Mona is working for Veronica Hastings' rival for State Senator, and is revealed to be Charlotte Drake's killer . </Li> <Li> Andrea Parker portrays Jessica DiLaurentis / Mary Drake, twin sisters . Jessica is the mother to Alison and Jason, while Mary is revealed to be the mother of Charlotte and Spencer . Mary was admitted to Radley Sanitarium for most of her life, but has now returned to Rosewood . During the fourth season, Mary kills Jessica and attempts to frame Peter Hastings for it . However, this is not revealed until season 7 . She comes back to Rosewood later, now allied with Archer Dunhill . In the nineteenth episode of season 7, she takes the blame for the death of Archer Dunhill so the Liars won't have to go to prison . In the season finale, it revealed that she escaped prison with the help of Mona so that she could ally with Alex Drake, her other biological daughter, Spencer's twin sister who is revealed as A.D </Li> <P> Rosewood is a fictional town in the United States state of Pennsylvania . It is the principal setting of the series and the books . The Liars live in this town along with most of the other characters . It contains many principal locations: the police station, the Rosewood High School, the church, the Rosewood Shopping Center, Hollis College, the Rosewood Community Hospital, the Rosewood movie theater, the mausoleum, the dental office, the Ophthalmology Cabinet, and the Rosewood Court . </P> <P> Virtually all these locations are actually exteriors on the backlot of the Warner Bros. studio lot in Burbank . Interior scenes are filmed separately on nearby soundstages . For example, different sides of the same building on the backlot are used for the entrances of the Rosewood police station and high school . </P>

What grade are the pretty little liars in on the show