The gallery’s Fifth Avenue and Chelsea venues are both given over to new works from China’s chief artist-provocateur. Uptown: 2,500 porcelain river crabs, their Chinese name evoking the government’s policy of “harmonization” (i.e., censorship), which prompted the destruction of Ai’s massive Shanghai studio. Downtown: twisted rebar from the rubble of the 2008 Sichuan earthquake that killed, among many others, 5,196 children in ill-constructed school buildings. Don’t miss the videos documenting Ai’s personal responses.