Digoxin and cardiac contractility

Digoxin inhibits the Na⁺/K⁺-ATPase in cardiac muscle and increases contraction strength. What is the mechanism?

1. AIt directly opens L-type Ca²⁺ channels✕
2. BIt increases sympathetic stimulation of β₁ receptors✕
3. CHigher intracellular Na⁺ slows Ca²⁺ removal by the Na⁺/Ca²⁺ exchanger, so more Ca²⁺ is available✕
4. DIt increases the sensitivity of troponin C to Ca²⁺✕