Combine the two ideas in the sentence.
Mountain climbing is hard. It requires a lot of practice.
Mountain climbing is a difficult activity that requires a substantial amount of practice.
Complete the following task based on the instruction and optional input provided.### Instruction:
Combine the two ideas in the sentence.

### Input:
Mountain climbing is hard. It requires a lot of practice.

### Output:
Mountain climbing is a difficult activity that requires a substantial amount of practice.