It was experimentally found that formic acid is stronger acid than acetic acid. Which of the following effects is useful to explain this?

Due to Inductive effect experimentally HCOOH is more acidic than. CH₃COOH.

Acidity increases if 
1. More minus I effect is present.
2. The resulting ion is stable.
3. The proton is donated fast.
4.Conjugate base must be stable.

The general order of acidity is,
Sulphonic acid>RCOOH>Phenol>Water>Alcohols>1,3 diketones>yne>ene>ane
Plus I effect decreases acidity.