Read the passage below and mark the letter A, B, C, or D on your answer sheet to indicate the best answer (A, B, C or D) that fits best for the gap.

In Sa Pa's Muong Hoa Valley, ancient carved stones reveal a rich history. Discovered in 1925 by a French scholar, over 30 carved stones were found along the Hoa Spring bank. Subsequent studies ___(1)___ more than 200 pieces, displaying intricate designs, ___(2)___ images of mountains, hills, fields, houses with boat-like structures, and curved roofs reminiscent of Dong Son bronze drums. Some stones hint at three distinct writing systems, resembling ___(3)___ of Tay and Dao ethnic groups, though not identical. The complexity of these findings makes it challenging to attribute the works ___(4)___ a specific author or ethnic group. The prevailing theories suggest diverse authors from various ethnic minorities, possibly including the ancient Viet people or neighboring groups. Legends persist about the "Father piece" and "Mother piece," linked to settlers at the mountain's base. ___(5)___ ongoing speculation, the true creators of these works remain a mystery.