Before revision:  Thus, each of the resources -- time, protein copy numbers, and energy -- limits sensing, and it is the limiting resource which places the fundamental limit on sensing .
After revision:  Thus, each of the resources -- time, protein copy numbers, and energy -- limits sensing, and it is the limiting class which sets the fundamental sensing limit. Within each class, trade-offs are possible. Energy allows a power-speed trade-off, while time can be traded against receptors .