Four cryogenic refrigerants are being evaluated for an aerospace thermal control system. Their melting and boiling points are listed in Table 1.
| Refrigerant | Melting point in ∘C^{\circ}\text{C}∘C | Boiling point in ∘C^{\circ}\text{C}∘C |
|---|---|---|
| P | -120 | -60 |
| Q | -30 | 150 |
| R | -80 | -10 |
| S | 25 | 300 |
Give the letter of the refrigerant that is a liquid at −40 ∘C-40\text{ }^{\circ}\text{C}−40 ∘C.
