R: How to write a function to do summation of a particular column values and report the row numbers when it meet certain criteria?

I am new to R and specially programming function.So, I would highly appreciate if you please try to help to understand the concepts.

I'd like to know how I can write a function to do the following acts:

1- add the values of certain variable(column) line by line (row by row) and 2- then if the sum of the value would be greater than 100; >100
3- then, stop and report all of the rows from begging till it meets the criteria (total sum <100) to stop?

As an example :

DT <- data.table(ID = c(1:21),Var1 = LETTERS[1:21], Var2 = seq(9, 19, .5))
> DT
    ID Var1 Var2
 1:  1    A  9.0
 2:  2    B  9.5
 3:  3    C 10.0
 4:  4    D 10.5
 5:  5    E 11.0
 6:  6    F 11.5
 7:  7    G 12.0
 8:  8    H 12.5
 9:  9    I 13.0
10: 10    J 13.5
11: 11    K 14.0
12: 12    L 14.5
13: 13    M 15.0
14: 14    N 15.5
15: 15    O 16.0
16: 16    P 16.5
17: 17    Q 17.0
18: 18    R 17.5
19: 19    S 18.0
20: 20    T 18.5
21: 21    U 19.0
    ID Var1 Var2

I'd like to add all of the value in Var2 row by row until the total sum would be > 100 then it stops and report all the rows from beginning till it meet this criteria.

I this case I mean, it add all of the values in Var2 from ID 1:9 (9.0 +9.5+10+10.5+11+11.5+12+12.5+13) which the total would be 99 and if add the next row (13.5) the values would be 112.5.

therefore the expected output would be :

 ID Var1 Var2
 1:  1    A  9.0
 2:  2    B  9.5
 3:  3    C 10.0
 4:  4    D 10.5
 5:  5    E 11.0
 6:  6    F 11.5
 7:  7    G 12.0
 8:  8    H 12.5
 9:  9    I 13.0

I tried different things , but to me they were not useful at all, hence, I am not including them here .