Challenge 3: Nullable vs. Plain Value Types — Possible Solution
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Working — Program.cs:

int? nullableCount = null;
Console.WriteLine(nullableCount);   // prints nothing / blank, no error

Broken attempt:

int plainCount = null;

Representative compile error:

    Program.cs(1,19): error CS0037: Cannot convert null to 'int'
    because it is a non-nullable value type

Explanation:
`int?` is really shorthand for Nullable<int> -- a wrapper struct that
adds an explicit "has a value or doesn't" flag on top of a real int,
letting the variable represent the absence of a value without
sacrificing int's own value-type semantics for the case where a value
IS present. A plain `int`, by contrast, always holds a real value
directly in the variable itself -- there is no separate "reference"
that could instead point to nothing. Assigning null to it makes no
sense at the type-system level, since null represents "no reference to
anything," and a plain int was never a reference to begin with -- so
the compiler rejects the assignment outright, at compile time, not
runtime.

WHY THIS WORKS AS AN ANSWER
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This reproduces the chapter's own int/int? distinction with a working
nullable example and the exact compile error a plain int produces,
and the explanation correctly ties the failure to value-type
semantics -- a plain value type holds real data directly, with no
reference-like "nothing" state to represent.