Difficulty: Easy
Correct Answer: AB * 2
Explanation:
Introduction:Classic Fortran identifier rules restrict variable names to specific character sets and formats. This question checks recognition of illegal characters and spacing in identifiers.
Given Data / Assumptions:
Concept / Approach:
Fortran variable names must begin with a letter and may contain letters and digits (and underscores in modern Fortran). Spaces and operator symbols (*, +, -, /) are not allowed inside identifiers. Therefore any token containing spaces or arithmetic symbols is invalid as a name.
Step-by-Step Solution:
Step 1: Check each option for illegal characters.Step 2: 'AB * 2' contains a space, an asterisk, and a digit separated by spaces — violates identifier rules.Step 3: 'BETA', 'ALPHA', and 'A' contain only letters and are valid.Verification / Alternative check:
Attempting to compile with AB * 2 as a variable name will be tokenized as identifier AB, operator *, literal 2 — a syntax error in declarations.
Why Other Options Are Wrong:
'BETA', 'ALPHA', and 'A' adhere to naming rules and compile successfully.
Common Pitfalls:
Confusing expression tokens with identifier characters; assuming spaces can be ignored inside names.
Final Answer:
AB * 2
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