以下关于数字摘要的叙述中，正确的是（ ）。

A．数字摘要采用了非对称加密技术

B．数字摘要是一串长度可变的信息串

C．数字摘要对于同一原文具有唯一性

D．数字摘要可以被还原得到原文信息

1. My grandfather left his hometown over 55 years ago.

My grandfather　_______________________________ for ____________ 55 years.

2. My grandparents got married 50 years ago.

It’s 50 years________________________________________________

3. My grandfather arrived in Nanjing last Monday. He is still in Nanjing.

My grandfather has　___________________ since last Monday.

4. My grandfather went to bed very late last night. He was too excited.

My grandfather_________________ go to bed__________ it was very late last night. He was too excited.

5. It took my grandfather two days to travel around the center of the city.

My grandfather　______________ two days______________________ the center of the city.