What is the main reason other members of the EU are concerned about the British decision to leave the EU? 

A. They are concerned that other nations will want to leave the EU after Britain.
B. They are concerned that English will not be spoken in the EU any longer.
C. They are concerned that Britain will join competing economic unions.*
D. They are concerned that Britain will no longer trade with EU nations.

I disagree.

Is it A

Yes, A.

The main reason why other members of the EU are concerned about the British decision to leave the EU is because they are concerned that Britain will join competing economic unions. This answer can be determined by understanding the implications of the Brexit decision.

To arrive at this answer:
1. Understand the context: The British decision to leave the EU is commonly known as "Brexit." It refers to the referendum held on June 23, 2016, in which the majority of British voters chose to leave the EU.
2. Consider the implications: When a country leaves the EU, it gains the freedom to negotiate and establish economic relationships with other countries or economic blocs. In the case of Brexit, there is concern among other EU members that Britain might seek to join other economic unions or trade agreements to replace its former ties with the EU.
3. Eliminate irrelevant options: Out of the given choices, options A and B are less relevant to the concerns of other EU members regarding Britain's decision to leave. Option D is also less likely as the EU and Britain are major trading partners, and severing those ties would have economic consequences for both parties.
4. Select the plausible answer: Option C, "They are concerned that Britain will join competing economic unions," emerges as the most reasonable answer as it aligns with the concerns of other EU members regarding the economic repercussions of Brexit.