Housed in the rectangular courtyard are the Hall of the Heavenly Emperor, Hall of the Great Heroes, and the Pilu Pavilion. Among these, the main structure is( ). A: Guanyin Pavilion B: the Hall of the Heavenly Emperor C: the Pilu Pavilion D: the Hall of the Great Heroes

Housed in the rectangular courtyard are the Hall of the Heavenly Emperor, Hall of the Great Heroes, and the Pilu Pavilion. Among these, the main structure is          .