______ is the second leading cause of death in the US, but the leading cause of smoking-related deaths.​

A) ​Cardiovascular disease
B) ​Cancer
C) ​Chronic lower respiratory disease
D) ​Mental illness

Cancer is the leading cause of smoking-related deaths and the second leading cause of death in the US. According to the Centers for Disease Control and Prevention (CDC), smoking causes about 1 in 3 cancer deaths in the US. Therefore, option B is the best choice.