12. (12 points) Fill in each box below with “ + ” or “ - ”, the sum of all different calculation results is ( )
A. 540
B. 600
C. 630
D. 650
【Solution】Solution: Since $25+9+7+5+3+1=50$, we conjecture that all even numbers between $0 \sim 50$ might be obtained. The sum of all even numbers from $0 \sim 50$ is $(0+50) \times 26 \div 2=650$;
When changing the + sign before 1 to a - sign, we get $25+9+7+5+3-1=48$, which is $1 \times 2$ less than 50,
When changing the + sign before 3 to a - sign, we get $25+9+7+5-3+1=44$, which is $3 \times 2$ less than 50,
When changing the + signs before 3 and 1 to - signs, we get $25+9+7+5-3-1=42$, which is $4 \times 2$ less than 50,
When changing the + sign before 5 to a - sign, we get $25+9+7-5+3+1=40$, which is $5 \times 2$ less than 50, ..
$22=1+5+7+9$, so when changing the + signs before $1,5,7,9$ to - signs, we get $25-9-7-5+3-1=6$, $24=3+5+7+9$, so when changing the + signs before $3,5,7,9$ to - signs, we get $25-9-7-5-3+1=2$, $25=1+3+5+7+9$, so when changing the + signs before $1,3,5,7,9$ to - signs, we get $25-9-7-5-3-1=0$,
Based on the above pattern, the numbers 2 and 23 cannot be formed, so the even numbers 4 and 46 cannot be obtained. Therefore, the answer is: $650-4-46=600$.
Hence, the correct choice is: $B$.