The British-designed warheads are thought to be selectable between 0.3 kilotons, 5–10 kt and 100 kt; the yields obtained using either the unboosted primary, the boosted primary, or the entire "physics package". The United Kingdom has purchased the rights to 58 missiles under the Polaris Sales Agreement (modified for Trident) from the United States Navy's "pool". These missiles are fitted with United Kingdom–built warheads and are exchanged when requiring maintenance. Under the agreement the United States was given certain assurances by the UK regarding the use of the missiles; however the United States does not have any veto on the use of British nuclear weapons. Some non-nuclear components for the British nuclear warhead are procured from the U.S. for reasons of cost effectiveness.