This program declares global bidimensional array of 30 rows and 5 columns (not counting 0 in the rows) of floating type to save information of students, table is as follows:

ID Partial1 Partial2 Partial3 Partial4 Average

ID: em..I
Partial N: Students grade of a evaluation.
Average: well students final grade.

the grade can't be higher than 10.

Ok so my problem is that when I start inputing data everything goes well until we input more than 1 student, when I enter the second ID it is saved but the second student's ID overwrites the last student's average, having the ID duplicated and the las average erased, and i can't figure out why is this happening, I've looked at the code for hours, so any help is really appreciated, by the way I'm relatively new to c++.

//Start of data input
    for (row= 1;row<=30;row++){
         Sum=0;//reset acumulator.
 
         printf("\nStudent %d.", row);
         printf("\n\tAverage: ");scanf("%f",&alumno[row][0]);
 
         for (col=1;col<=4;col++){
              printf("\n\tPartial %d: ",col);scanf("%f",&alumno[row][col]);
              Sum=Sum+alumno[row][col];//acumulating values for average
         }
 
         alumno[row][5]=Sum/4; //this is the average
         printf("\nAverage: %0.2f", alumno[row][5]);
    }//end of input

ID  P1 P2 P3 P4 Av
10 10 10 10 10 9
9   9    9  9   9   6
6   6    6  6   6   6