这道题的正确答案为D therefore.
However,意为无论如何,颇有些无奈的意思.举例来讲,I don't like my docotr,but however,I have to listen to him.我不喜欢我的医生,但没有办法,我必须得听他的.
Otherwise,意为要不然,这个就有一种庆幸的味道在里面了.For exmple:Thank goodness that I passed the test,otherwise I have to stay after school.谢天谢地我通过了这次考试,要不然我放学之后还要留在学校学习.
Still,意为仍然,一种类似于转折的语气.It was still raining when I get home.我回家的时候雨还在下.
Therefore,意为所以,有一种说教的意味在里面.helping the old and the weak is a good virtue and( therefore )it is necessary for us to do so 帮助老弱病残是一种美德,所以我们需要这样做.
一般来说,如果前半句为explanation,多半后半句要用therefore.