Can someone please answer this question (the question is attached)

Iknow the answer is a but i don't know why

Needthe meter is the standard unit for: 1) height 2) length 3) weight 4) mass

Substance a is a nonpolar liquid and has only dispersion forces among its constituent particles. substance b is also a nonpolar liquid and has about the same magnitude of dispersion forces among its constituent particles. when substance a and b are combined, they spontaneously mix.