You are operating an IoT solution which collects sensor data from a wide range of field devices, including speed sensors, cameras etc. The devices are connected to an S2 hub. In order to prevent data loss and throttling, you want to set up alerts to warn you when the message throughputfor the last 1 minute period exceeds the 80% of the throttle threshold. 
Which metric alert should you set up in your IoT Hub?

Answer options:

A.Telemetry messages sent > 10000 / min
B.Telemetry messages sent > 6000 / min
C.Telemetry message send attempts > 10000 / min
D.Total number of messages used > 6000 / min

Correct Answer: B
Option A is incorrectbecause for S2 hubs, the flow of device-to-cloud messages will be automatically throttled when the flow rate reaches 120 messages/s/unit. Setting the threshold value to 10 000 (i.e. 160 messages/s ) will not prevent throttling.
Option B is CORRECTbecause for S2 IoT hubs, the number of device-to-cloud messages is limited to 120/sec/unit. It means that at this rate the automatic throttling takes effect. In order to fulfil the requirement, you need to use the Telemetry messages sent metric to 6 000 messages per minute (i.e. 100 messages/s, close to the 80% of the threshold value).
Option C is incorrect because the throttling logic is triggered by the number of messages successfully sent to the IoT Hub, which is measured by the Telemetry messages sent metric.
Option D is incorrect because the Total number of messages used metric is related to the quotas. It is the aggregate number of messages you can send in your hub per day. It is not suitable for monitoring the send rate of the message flow.
References: 

https://docs.microsoft.com/en-us/azure/iot-hub/iot-hub-devguide-quotas-throttling#operation-throttles
https://docs.microsoft.com/en-us/azure/iot-hub/tutorial-use-metrics-and-diags#set-up-metric-alerts