You need to set up an IoT infrastructure and you need to determine what type and number of IoT hubs to provision. You need to connect 1000 devices, each sending 6 telemetry messages per minute, and the devices expected to receive 10 messages per each from the IoT hub daily. 
How many units of IoT hubs do you need to provision and of what type?

Answer options:

A.1 unit of B2
B.2 units of B2
C.2 units of S2
D.1 unit of S1

Correct Answer: C
Option A is incorrect because you need to send cloud-to-device messages, and this feature is only available in the S (Standard) tier hubs.
Option B is incorrect becauseyou need to send cloud-to-device messages, and this feature is only available in the S (Standard) tier hubs.
Option C is CORRECT because you need to send C2D messages, which is only available in S (Standard) tier. The expected send rate of your devices is 1000*6=6000 messages/min. One unit of size 2 hub is able to handle 4167 messages/minute, therefore you’ll need 2 of them.
Option D is incorrect because one unit of a size 1 hub can only handle an average of 278 messages/min, therefore it cannot cope with the expected send rate.
References: 

https://docs.microsoft.com/en-us/azure/iot-hub/iot-hub-scaling#message-throughput
https://docs.microsoft.com/en-us/azure/iot-hub/iot-hub-devguide-quotas-throttling