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Scientists determined the proteome of two strains, strain 1 and strain 2, of a pathogenic bacterium. They want a single vaccine that protects against both strains. The table shows four surface proteins and whether each is produced by each strain.
| Surface protein | Produced by strain 1 | Produced by strain 2 |
|---|---|---|
| A | Yes | No |
| B | Yes | Yes |
| C | No | Yes |
| D | Yes | Yes |
Using the table, identify the two proteins that could be used as antigens in a vaccine effective against both strains.
Explain why proteins A and C would not be suitable for a vaccine that must protect against both strains.