What is the explanation offered by the liquidity preference theory for the upward sloping yield...

The long term rates must rise enough to get some borrowers to borrow short-term and some lenders to lend long-term.

The long term rates must rise enough to get some borrowers to borrow long-term and some lenders to lend short-term.

The short term rates must rise enough to get some borrowers to borrow short-term and some lenders to lend long-term.

The short term rates must fall enough to get some borrowers to borrow long-term and some lenders to lend short-term.