a drop of blood的用法和样例：
- A drop of blood slid down his leg.
- A drop of blood slid downhis leg.
- I am afraid I have to prick your finger and take a drop of blood.
- I would like to take a drop of blood from his ear for a blood test.
- At every jolt over the pavement, a drop of blood trickled from Marius' hair.
- He made up his mind never to touch a drop of drink.