A student was asked to name an element that reacts like chlorine (Cl). Which element should the student name?

The population of unionid mussels decreased after zebra mussels were introduced. zebra mussels competed with the unionid mussels for food. the zebra mussels were able to displace the unionid mussels, allowing the zebra mussel population to increase  over the years. the unionid mussel population decreased because  of lack of food.

H tricky one let me think and get back to u