Which of the following $p-n$ junction devices is not used in reverse bias?

In which of the following cases is the junction diode $NOT$ in reverse bias?

$A$ thermoelectric refrigerator works on

When a potential difference is applied across,the current passing through:

In an oscillator,for sustained oscillations,the Barkhausen criterion is $A \beta$ equal to (where $A =$ voltage gain without feedback,$\beta =$ feedback factor).

The figure shows an intrinsic semiconductor $S$ connected in series with a variable resistor $R$ and a constant voltage source $V$. The value of $R$ can be adjusted to maintain a constant current in the ammeter $A$. Under what condition will the semiconductor $S$ get heated? Give reasons.