sole

「sole」是「僅有、唯一的」，例如：

My sole reason for visiting here was to see Chiang Kai-shek Memorial Hall.（我來這裡的唯一理由，就是參觀中正紀念堂。）

That will be the sole means of access to the building.（那是進入這棟建築物的唯一途徑。）

Claire was the sole survivor of the terrible accident.（克萊兒是這可怕意外事故的唯一倖存者。）

hear a pin drop

「could hear a pin drop」是「鴉雀無聲、 完全安靜，幾乎是『可以聽見針掉落的聲音』」，例如：

As the audience waited for the Nobel Prize winner to speak, you could even hear a pin drop.（聽眾在等待諾貝爾得獎者發表感言時，甚至可以聽到針掉落的聲音。）

After the teacher's outburst, the classroom was so quiet you could hear a pin drop.（老師大發一頓脾氣之後，教室鴉雀無聲。）

It was completely silent before the piano played in front of several thousand people. You could hear a pin drop.（在千人鋼琴彈奏會開始之前，可以說是完全的寂靜。）