Final answer:
In the tactical shooter game, Viper's Toxic Screen (E) ability is available to use each round at no credit cost, instead relying on a replenishable fuel resource to function.
Step-by-step explanation:
Viper's Toxic Screen (E) ability in the popular tactical shooter game does not have a credit cost to use. As a controller agent, Viper uses her Toxic Screen to create a line of toxic gas emitters that can be activated to create a tall wall of poison gas, which obstructs vision and damages enemies who pass through it. Viper's abilities are designed to control the battlefield and her Toxic Screen is available each round without a purchase, although it relies on her fuel resource to be activated and sustained.