A man was charged with the sale of narcotics.  The federal prosecutor arranged with the man's wife for her to testify against her husband in exchange for leniency in her case.  At trial, the prosecution calls the wife, 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 federal courts?

选项：

A:

The wife can be called as a witness but cannot testify, over her husband's objection, that she saw him sell heroin.

B:

The wife can be required to be a witness and to testify that she saw her husband sell heroin.

C:

The wife can refuse to be a witness against her husband.

D:

The wife cannot be called as a witness over her husband's objection.