have to与will have to稍有不同

有读者问一则改错题:

If the volume of British investment is (A: to be) increased, (B: other) demands on national resources (C: were having to) be reduced or (D: eliminated).

答案为C错,应改为have to。我认为应改为will have to。请赐教。

have to与will have to都不错,皆可表未来。但have to似乎具有“必然”的含义。