To be sure. selectivity may be exercised under. seetion. 212(a) (28) in determining which of the aliens falling within the. categories enumerated therein are to be admitted under waivers and which are to be excluded. Section 212 (a) (27) and 212(a) (29). however. are more directly aimed at preventing the entry of individuals who present any substantial threat to the national security. Subsection (27)prohibits the issuance of a visa to any alien who the consular officer or Attorney General has reason to believe is entering "solely. principally. or incidentally to engage in activities which would be prejudicial to the public interest. or endanger the welfare. safety. or security of the United States." Subsection (29) bars entry to all aliens who the consular officer or Attorney General has reason to believe would engage In activity "subversive to the national security" or who would "participate in the activities" of any subversive organization. These specific provisions. which impose an ineligibility that may not be waived. appear to provide ample protection for the national security without affecting. as section 212(a) (28) does. the travel of a host of Individuals whose purpose is entirely innocent and whom we would welcome to this country as tourists or businessmen. To prevent people from entering this country on the basis of what they believe. or have believed in the past.