You are setting up a new data center for your company, which requires high$-$speed data transmission and the flexibility to connect devices using both fiber optic and copper cables. To achieve this, you decide to use modular transceivers in your network equipment.

One of your primary goals is to ensure that the network can handle $40$ Gigabit Ethernet $(40$GbE $)$ over fiber optic cables for connections between core switches and aggregation switches. Additionally, you want the capability to use copper cables for shorter, less critical connections without changing the physical hardware.

Given these requirements, which type of modular transceiver would you primarily need to use for the fiber optic connections between the core and aggregation switches?

SFP transceivers

QSFP$+$ transceivers

WDM transceivers

DAC ports