bothers me is that I can't keep my mind on the task, but on personal gains or losses.

What

It

That

One

考查主语从句连接词。句意：困扰我的是我不能专注于任务，而是专注于个人得失。此处为主语从句，从句中缺少主语，结合句意表示“困扰我的是”应用what。故选A项。

The teacher was so pleased with the progress that I had        in my study this term that she sent me an English-Chinese dictionary.

made

had 

kept

graded

考查动词辨析。句意：老师对我在这学期在学习上取得的进步非常满意，寄给我一本英汉词典。分析可知hat I had        in my study this term 应是之前名词the progress的定语从句，先行词在从句中属于固定结构make progress“取得进步”的宾语。故选A项。

The most important promises we must keep are          we make to ourselves.

ones

those

these

them

考查代词。句意：我们必须遵守的最重要的承诺就是我们对自己做出的承诺。A. ones泛指上文提到的可数名词复数；B. those特指上文提到的可数名词复数；C. these这些；D.them他们。分析句子可知，为we must 是定语从句，修饰The most important promises，是句子的主语；be动词后缺少表演，we make to ourselves是定语从句，修饰空格处。根据句意，此处指上文提到的promises,由定语从句修饰表示特指，故用 those。故选B项。

Writing out all the invitations by hand was more time-consuing than we             .

will except

are  excepting

except

had excepted

考查时态。句意：手工写下所有的邀请函要比我们预料的花去更多的时间。根据句意可知我们预料是在实际上写之前发生的，而本句中实际写花去的时间使用了过去时was，所以expect用过去完成时。故D项正确。

It was along the Mississippi River _________ Mark Twain spent much of his childhood.

how

which

that

where

此处构成It was...that...强调句式，句中强调了介词短语along the Mississippi River。故选C。