C139 Deep feeling

"Eleven, have you ever studied whether the charcoal and saltpeter contained anything like what you were talking about?"

Situ Jing seemed to have thought of something, and immediately questioned Gu Shiyi.

Gu Shiyi shook her head, and smiled at Situ Jing, as they thought of the same thing.

"So, the problem is sulfur!"

"Indeed