Mr. Denby was charged with the sale of narcotics. The federal prosecutor arranged with Mrs. Denby for her to testify against her husband in exchange for leniency in her case. At trial, the prosecution calls Mrs. Denby, who had been granted immunity from prosecution, to testify, among other things, that she saw her husband sell an ounce of heroin. Which of the following statements is most clearly correct in the federal courts? 
Mrs. Denby cannot be called as a witness over her husband's objection. Mrs. Denby can be called as a witness but cannot testify, over Mr. Denby's objection, that she saw him sell heroin.  Mrs. Denby can refuse to be a witness against her husband. Mrs. Denby can be required to be a witness and to testify that she saw her husband sell heroin.