PCA (Prometheus Certified Associate) sample exam question with answer 256

Question:
Consider the following histogram bucket time series and assume the existence of the corresponding http_request_duration_seconds_sum and http_request_duration_seconds_count series. Also assume a scrape interval of 1m.
What is the result of the query given below?
Series:

http_request_duration_seconds_bucket{le="1"}    0 0 0 0 1
http_request_duration_seconds_bucket{le="2"}    1 1 2 4 5
http_request_duration_seconds_bucket{le="5"}    2 2 3 4 6
http_request_duration_seconds_bucket{le="+Inf"} 3 5 5 7 9

Query: http_request_duration_seconds_count

1. {} 9
2. {} 6
3. http_request_duration_seconds_count 6
4. http_request_duration_seconds_count 9

Answer:
D - the resulting value should be 9 since the histogram *_count metric always matches the value held by the +Inf bucket. Also the result will include metric name