For your first question, the correct response is:
D Trench warfare developed due to the advancements made in artillery that occurred before the development of advanced military mobility.
Trench warfare became prominent during World War I primarily because of the increased firepower and effectiveness of artillery. The sheer destructive capability of heavy artillery made traditional advances difficult and led to soldiers digging in to protect themselves.
For your second question, the correct response is:
C The invulnerability of the tank to machine gun and rifle attack decreased the tactic.
The introduction of tanks during World War I allowed for effective movement across the battlefield, overcoming the stalemate of trench warfare by being able to cross difficult terrain and break through fortified positions, thus diminishing the effectiveness of entrenched positions.