Answer:
D) had been trained over the past two years and that they had
Step-by-step explanation:
In the context, the sentence speaks of something that happened in the past, so 'had been' is required and also it speaks of parallelism of events, as an estimation is being made so, Â parallelism should be maintained.
Hence, option (D) satisfies the two requirements. And eliminate option A, B, C and E.
Thus, (D) is correct.