For Kiera, she pays just $25 per month.
So the amount paid for m months will be x * 25 = $25m
For Trina, she pays a $100 fee for joining and $10 per month
so the amount she pays after m months will be 10(m) + 100 = 10m + 100
Hence;
t1 = 25m
t2 = 10m + 100
Option C is the correct answer