Use of Sharpe Ratio for fund managers

CFA Level II Portfolio Management
#1

We are least likely to use the Sharpe ratio to evaluate the ex post portfolio returns of which of the following managers?

  1. Manager 1: A fund holding stocks of companies located in the major European markets. Its benchmark is the FTSE Developed Europe Index.
  2. Manager 2: A fund with a total return objective of exceeding the return of the FTSE Developed Europe Index Fund. The fund can hold up to 20% in cash.
  3. Manager 3: A fund that holds eurozone sovereign bonds. Its benchmark is the S&P Eurozone Sovereign Bond 10+ Years Index.

The answer is (2) - I understand that it is because the SR is unaffected by cash.

But why is (3) wrong? If the fund holds sovereign bonds, wouldn’t R_p be very close to R_f ? How is the SR even meaningful here?