Silver is a white metal and is an excellent conductor of heat and electricity. The density of silver is 10.49 g/cm³. Silver does not react with water, but it does react with nitric acid. Silver tarnishes when exposed to air or light. Which is a physical property of silver?

The first sentence is a physical property. This is a physical property because it does not involve a chemical reaction only a physical observation

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.