Answer:
321Go
Explanation:
The identifiers in C, C++, C#, Java and other programming languages are a combination of letters, numbers and the underscore symbol. The laters versions of C and C++ allows you to use almost all Unicode characters. In Java, you can use also the dollar sign.
From that you have to be careful to follow these rules:
-Don't use keywords.
-Don't include white spaces.
-Don't use operators.
-Don't repeat identifiers.
-Don't start your identifier with a number.
-Don't use two consecutive underscores.
So app_234, happyTimesAhead, and cis22B are follo wing these rules but 321Go starts with a number.
