Why is strontium more reactive than calcium but less reactive than rubidium?

Sr is  below Ca and to the right of Rb in the Periodic Table (see the attached image).

Reactivity increases from top to bottom of a Group. Sr is below Ca, so it is more reactive than Ca.

Reactivity decreases from left to right in a Period. Sr is to the right of Rb, so it is less reactive than Rb.

a:

by the combination of an unlimited number of elements

answer: any goods being imported from another country may be subject to issued by the federal government are tariffs

explanation:  

tariffs are a tax on anything that are imported from abroad. government use them to protect and boost their local industries and the thought is that they encourage consumers to buy local instead of importing and they will hope will protect jobs and economy in the country that implements them