Underlying Representation

I'm really confused about how to determine underlying representation. Everything I read seems to contradict the last.

Trying desperately to solve this problem and I just seem to be going in circles with both 1 and 2. Any suggestions?

In Maga Rukai, an Austronesian language spoken in Taiwan, certain prefixes show an alternation between a mid vowel and a high vowel.
1) For each alternation, state the underlying representation. Justify.
2) What determines whether the prefix vowel is mid or high? State the rule that would describe this.

o ~ u dynamic verb marker
o-kamɨ ‘back’ u-cŋulu ‘connect’
o-dranɨ ‘make a road’ u-ɖmiŋi ‘make a road’
o-lapɨ ‘hunt without dogs’ u-lupu ‘hunt with dogs’

i ~ e negative marker
i-tukruː ‘not hit with stones’ e-kapliː ‘not hold with hands’
i-ulpuː ‘not hunt’ (with dogs) e-lalpɨː ‘not hunt’ (without dogs)

si ~ se to wear
si-slivi ‘wear beads’ se-kceŋe ‘wear pants’
si-krɨkrɨ ‘wear a necklace’ se-kcabu ‘wear leggings’

te ~ ti to make
te-kceŋe ‘make pants’ ti-slivi ‘make beads’
te-tovnaː ‘build a hut’ ti-kunu ‘make a skirt’
te-sdamraː ‘cook a side dish’ ti-krɨkrɨ ‘make a necklace’

ke ~ ki harvest
ke-teθo ‘harvest turnip’ ki-sito ‘harvest peanut’
ke-bləblə ‘harvest bamboo shoot’ ki-lpɨlpɨ ‘harvest peas’

ke ~ ki passive
ke-ɖoɖoː ‘be awaited’ ki-klukluɖu ‘be frightened’
ke-kθabɨ ‘peeled’ ki-tɨtɨ ‘be bitten’