Could you please check these three sentences, please?

Question

1) Describe the kind of technique adopted by 20-century writers to represent the unspoken activity of the mind. Provide an example of its use from the readings done in class.
2) I had to authenticate all the students' tests and check that they had handed all their examination papers.
3) The First World War is thought to have brought cataclysmic changes in life in the whole of Europe.

Answer 1

2) that they had handed in OR submitted

Sra

Answer 2

"20-century writers" - very non-standard

This should be written 20th century ...