Answer 1

Using the uninitialized variable in expressions is considered undefined behaviour ¹
. It might be 0 on SoloLearn or a meaningless value like -123456789 on a workstation depends on the kind of variable that has been declared (the acceptable range of the garbage value) and the OS. So, you'd always want to make sure that all of the declared variables get initialized to a meaningful initial values (is always considered a good programming practice).

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¹
https://en.m.wikipedia.org/wiki/Undefined_behavior

Answer 2

you dont need to print score at the first printf, and it's not yet initialize