Answer:
A) are possible because proportional increases in inputs yielding the same proportional increase in output may induce higher input prices.
Step-by-step explanation:
Constant returns to scale mean that any proportional increase in inputs will result in an equally proportional increase in outputs.
The price of inputs might also rise because their supply curves are also upward sloping. This would result in an increasing cost industry, that will have an upward sloping long run supply curve.
So an industry can have constant returns to scale and upward sloping supply curve.