I messed up on my grades badly?

Today my grandma visited me and said she would buy me an iphone if all of my grades were A's and B's but I have one C all of you are probably saying it's not a big deal but in my family good grades are really important B's arent that appreciated in my family. Although there's a good reason I got a C in that class I still don't think she will just ignore the C even with my excuse I do have a 100% in one class maybe I could use that to let her give me an iphone.

my grades
Algebra. + 86%
LA. - 81%
STEM: A+ 100%
Science. - 82%
Social Studies: C 70%

Added (1). just-my-opinion schools, districts, and or states can have different grading systems at my school 70-79 is a C 80-89 is a B 90-100 is an A

Just tell your grandma why you have a C in stoical studies. If it is a good reason, she should understand. This is my first ever answer, so it might not be good.

You did well overall, but clearly this is between you and your gran.

Those are amazing results, you should be proud! Maybe you could save for your iphone; that way you could prove to your family that you not only get decent grades, but that you can save for the things you want and show independence.

Where do you go to school that a 70 equals a C - around here that would be a D. And several of those Bs that you have would be Cs here.

If Grandma said As and Bs that's what she meant. Excuses will get you nowhere in life.

Maybe you can get all As & Bs next semester. This will be great incentive for you to accomplish that.

EDITED TO ADD THIS AFTER YOU ADDED MORE INFORMATION:
Yes, I know that, but that is a very generous grading scale, so with that you should be able to do all As and Bs