In vitro bioaccessibility testing is gaining popularity as a tool to estimate the oral bioavailability of contaminants in soil for human health risk assessment (HHRA). Bioaccessibility tests are used to measure the bioaccessible fraction of a contaminant in soil, which can then be used to estimate the bioavailable fraction.上文是文献里面的句子，，1.请教这两个单词的精确区别 bioavailability和bioaccessibility？ 2.oral bioavailability 是什么意思啊？