PCA (Prometheus Certified Associate) sample exam question with answer 555

Question:
Given the following two time series and assuming a scrape interval of 1m, what is the result of the query given below?
Series:

up{job="node", instance="node1"} 0 1 1 0 NaN
up{job="node", instance="node2"} 1 0 1 1 0

Query:

max_over_time(up[5m])

1. up{job="node", instance="node1"} 1
   up{job="node", instance="node2"} 1

2. {job="node", instance="node1"} 1
   {job="node", instance="node2"} 1

3. {job="node", instance="node1"} NaN
   {job="node", instance="node2"} 1

4. {job="node"} 0

Answer:
B - is the correct answer. More at aggregation_over_time
A - is incorrect as temporal aggregations like max_over_time drop the metric name from the results they produce
C - is incorrect as the max_over_time aggregation only returns NaN if all values in the range interval are NaN
D - is incorrect as temporal aggregations maintain all of the input series as results. This would have been the correct answer if the query was max(up) by (job)