You are using Google BigQuery as your data warehouse. Your users report that the following simple query is running very slowly, no matter when they run the query:
SELECT country, state, city FROM [myproject:mydataset.mytable] GROUP BY country
You check the query plan for the query and see the following output in the Read section of Stage:1:

What is the most likely cause of the delay for this query?

A. Users are running too many concurrent queries in the system B. The [myproject:mydataset.mytable] table has too many partitions C. Either the state or the city columns in the [myproject:mydataset.mytable] table have too many NULL values D. Most rows in the [myproject:mydataset.mytable] table have the same value in the country column, causing data skew