Three sentences with preposition before "which":

1. I was surprised by the speed at which he completed the project.
2. The rate at which trees absorb CO2 varies by species.
3. The degree to which they agreed was astonishing.

Two sentences without:

1. I read the book, which was very interesting.
2. They’re selling the house which they bought just last year.

Can I say these?

1. I was surprised by the speed which he completed the project.
2. I was surprised by the speed that he completed the project.
3. They’re selling the house for which they bought just last year.

Thanks in advance for explanation of the rules.